diff --git a/build.sbt b/build.sbt
index bc2cf4cb0..7cf968a89 100644
--- a/build.sbt
+++ b/build.sbt
@@ -12,7 +12,7 @@ import Sxr.sxr
 // but can be shared across the multi projects.
 def buildLevelSettings: Seq[Setting[_]] = Seq(
   organization in ThisBuild := "org.scala-sbt",
-  version in ThisBuild := "0.13.8-SNAPSHOT"
+  version in ThisBuild := "0.13.9-SNAPSHOT"
 )
 
 def commonSettings: Seq[Setting[_]] = Seq(
@@ -522,7 +522,7 @@ def otherRootSettings = Seq(
   }
 ))
 lazy val docProjects: ScopeFilter = ScopeFilter(
-  inAnyProject -- inProjects(root, sbtProj, scriptedBaseProj, scriptedSbtProj, scriptedPluginProj, precompiled282, precompiled292, precompiled293),
+  inAnyProject -- inProjects(root, sbtProj, scriptedBaseProj, scriptedSbtProj, scriptedPluginProj, precompiled282, precompiled292, precompiled293, mavenResolverPluginProj),
   inConfigurations(Compile)
 )
 def fullDocSettings = Util.baseScalacOptions ++ Docs.settings ++ Sxr.settings ++ Seq(
diff --git a/compile/integration/src/main/scala/sbt/compiler/AggressiveCompile.scala b/compile/integration/src/main/scala/sbt/compiler/AggressiveCompile.scala
index c25b0673d..201e6108d 100644
--- a/compile/integration/src/main/scala/sbt/compiler/AggressiveCompile.scala
+++ b/compile/integration/src/main/scala/sbt/compiler/AggressiveCompile.scala
@@ -149,9 +149,12 @@ class AggressiveCompile(cacheFile: File) {
           // previous Analysis completely and start with empty Analysis object
           // that supports the particular value of the `nameHashing` flag.
           // Otherwise we'll be getting UnsupportedOperationExceptions
+          log.warn("Ignoring previous analysis due to incompatible nameHashing setting.")
           Analysis.empty(currentSetup.nameHashing)
         case Some(previous) if equiv.equiv(previous, currentSetup) => previousAnalysis
-        case _ => Incremental.prune(sourcesSet, previousAnalysis)
+        case _ =>
+          log.warn("Pruning sources from previous analysis, due to incompatible CompileSetup.")
+          Incremental.prune(sourcesSet, previousAnalysis)
       }
       IncrementalCompile(sourcesSet, entry, compile0, analysis, getAnalysis, output, log, incOptions)
     }
diff --git a/ivy/src/main/scala/sbt/MakePom.scala b/ivy/src/main/scala/sbt/MakePom.scala
index dde1f40ef..de75756cd 100644
--- a/ivy/src/main/scala/sbt/MakePom.scala
+++ b/ivy/src/main/scala/sbt/MakePom.scala
@@ -109,13 +109,15 @@ class MakePom(val log: Logger) {
     {
       if (moduleInfo.developers.nonEmpty) {
         <developers>
-          moduleInfo.developers.map{ developer: Developer =>
-            <developer>
-              <id>{ developer.id }</id>
-              <name>{ developer.name }</name>
-              <email>{ developer.email }</email>
-              <url>{ developer.url }</url>
-            </developer>
+          {
+            moduleInfo.developers.map { developer: Developer =>
+              <developer>
+                <id>{ developer.id }</id>
+                <name>{ developer.name }</name>
+                <email>{ developer.email }</email>
+                <url>{ developer.url }</url>
+              </developer>
+            }
           }
         </developers>
       } else NodeSeq.Empty
diff --git a/main/settings/src/main/scala/sbt/Remove.scala b/main/settings/src/main/scala/sbt/Remove.scala
new file mode 100644
index 000000000..a08093126
--- /dev/null
+++ b/main/settings/src/main/scala/sbt/Remove.scala
@@ -0,0 +1,20 @@
+package sbt
+
+import scala.annotation.implicitNotFound
+
+object Remove {
+  @implicitNotFound(msg = "No implicit for Remove.Value[${A}, ${B}] found,\n  so ${B} cannot be removed from ${A}")
+  trait Value[A, B] extends Any {
+    def removeValue(a: A, b: B): A
+  }
+  @implicitNotFound(msg = "No implicit for Remove.Values[${A}, ${B}] found,\n  so ${B} cannot be removed from ${A}")
+  trait Values[A, -B] extends Any {
+    def removeValues(a: A, b: B): A
+  }
+  sealed trait Sequence[A, -B, T] extends Value[A, T] with Values[A, B]
+
+  implicit def removeSeq[T, V <: T]: Sequence[Seq[T], Seq[V], V] = new Sequence[Seq[T], Seq[V], V] {
+    def removeValue(a: Seq[T], b: V): Seq[T] = a filterNot b.==
+    def removeValues(a: Seq[T], b: Seq[V]): Seq[T] = a diff b
+  }
+}
diff --git a/main/settings/src/main/scala/sbt/Structure.scala b/main/settings/src/main/scala/sbt/Structure.scala
index 153f48e7c..56e08e2d2 100644
--- a/main/settings/src/main/scala/sbt/Structure.scala
+++ b/main/settings/src/main/scala/sbt/Structure.scala
@@ -42,13 +42,19 @@ sealed abstract class SettingKey[T] extends ScopedTaskable[T] with KeyedInitiali
   final def ++=[U](vs: U)(implicit a: Append.Values[T, U]): Setting[T] = macro std.TaskMacro.settingAppendNImpl[T, U]
   final def <+=[V](v: Initialize[V])(implicit a: Append.Value[T, V]): Setting[T] = macro std.TaskMacro.settingAppend1Position[T, V]
   final def <++=[V](vs: Initialize[V])(implicit a: Append.Values[T, V]): Setting[T] = macro std.TaskMacro.settingAppendNPosition[T, V]
+  final def -=[U](v: U)(implicit r: Remove.Value[T, U]): Setting[T] = macro std.TaskMacro.settingRemove1Impl[T, U]
+  final def --=[U](vs: U)(implicit r: Remove.Values[T, U]): Setting[T] = macro std.TaskMacro.settingRemoveNImpl[T, U]
   final def ~=(f: T => T): Setting[T] = macro std.TaskMacro.settingTransformPosition[T]
-  final def transform(f: T => T, source: SourcePosition): Setting[T] = set(scopedKey(f), source)
 
   final def append1[V](v: Initialize[V], source: SourcePosition)(implicit a: Append.Value[T, V]): Setting[T] = make(v, source)(a.appendValue)
   final def appendN[V](vs: Initialize[V], source: SourcePosition)(implicit a: Append.Values[T, V]): Setting[T] = make(vs, source)(a.appendValues)
 
-  protected[this] def make[S](other: Initialize[S], source: SourcePosition)(f: (T, S) => T): Setting[T] = this.set((this, other)(f), source)
+  final def remove1[V](v: Initialize[V], source: SourcePosition)(implicit r: Remove.Value[T, V]): Setting[T] = make(v, source)(r.removeValue)
+  final def removeN[V](vs: Initialize[V], source: SourcePosition)(implicit r: Remove.Values[T, V]): Setting[T] = make(vs, source)(r.removeValues)
+
+  final def transform(f: T => T, source: SourcePosition): Setting[T] = set(scopedKey(f), source)
+
+  protected[this] def make[S](other: Initialize[S], source: SourcePosition)(f: (T, S) => T): Setting[T] = set((this, other)(f), source)
 }
 
 /**
@@ -67,10 +73,15 @@ sealed abstract class TaskKey[T] extends ScopedTaskable[T] with KeyedInitialize[
   def ++=[U](vs: U)(implicit a: Append.Values[T, U]): Setting[Task[T]] = macro std.TaskMacro.taskAppendNImpl[T, U]
   def <+=[V](v: Initialize[Task[V]])(implicit a: Append.Value[T, V]): Setting[Task[T]] = macro std.TaskMacro.taskAppend1Position[T, V]
   def <++=[V](vs: Initialize[Task[V]])(implicit a: Append.Values[T, V]): Setting[Task[T]] = macro std.TaskMacro.taskAppendNPosition[T, V]
+  final def -=[U](v: U)(implicit r: Remove.Value[T, U]): Setting[Task[T]] = macro std.TaskMacro.taskRemove1Impl[T, U]
+  final def --=[U](vs: U)(implicit r: Remove.Values[T, U]): Setting[Task[T]] = macro std.TaskMacro.taskRemoveNImpl[T, U]
 
   def append1[V](v: Initialize[Task[V]], source: SourcePosition)(implicit a: Append.Value[T, V]): Setting[Task[T]] = make(v, source)(a.appendValue)
   def appendN[V](vs: Initialize[Task[V]], source: SourcePosition)(implicit a: Append.Values[T, V]): Setting[Task[T]] = make(vs, source)(a.appendValues)
 
+  final def remove1[V](v: Initialize[Task[V]], source: SourcePosition)(implicit r: Remove.Value[T, V]): Setting[Task[T]] = make(v, source)(r.removeValue)
+  final def removeN[V](vs: Initialize[Task[V]], source: SourcePosition)(implicit r: Remove.Values[T, V]): Setting[Task[T]] = make(vs, source)(r.removeValues)
+
   private[this] def make[S](other: Initialize[Task[S]], source: SourcePosition)(f: (T, S) => T): Setting[Task[T]] =
     set((this, other) { (a, b) => (a, b) map f.tupled }, source)
 }
diff --git a/main/settings/src/main/scala/sbt/std/TaskMacro.scala b/main/settings/src/main/scala/sbt/std/TaskMacro.scala
index 51f6ed028..770eea263 100644
--- a/main/settings/src/main/scala/sbt/std/TaskMacro.scala
+++ b/main/settings/src/main/scala/sbt/std/TaskMacro.scala
@@ -67,6 +67,8 @@ object TaskMacro {
   final val AssignInitName = "set"
   final val Append1InitName = "append1"
   final val AppendNInitName = "appendN"
+  final val Remove1InitName = "remove1"
+  final val RemoveNInitName = "removeN"
   final val TransformInitName = "transform"
   final val InputTaskCreateDynName = "createDyn"
   final val InputTaskCreateFreeName = "createFree"
@@ -136,29 +138,58 @@ object TaskMacro {
   def taskAppend1Impl[T: c.WeakTypeTag, U: c.WeakTypeTag](c: Context)(v: c.Expr[U])(a: c.Expr[Append.Value[T, U]]): c.Expr[Setting[Task[T]]] =
     {
       val init = taskMacroImpl[U](c)(v)
-      val assign = appendMacroImpl(c)(init.tree, a.tree)(Append1InitName)
-      c.Expr[Setting[Task[T]]](assign)
+      val append = appendMacroImpl(c)(init.tree, a.tree)(Append1InitName)
+      c.Expr[Setting[Task[T]]](append)
     }
   /** Implementation of += macro for settings. */
   def settingAppend1Impl[T: c.WeakTypeTag, U: c.WeakTypeTag](c: Context)(v: c.Expr[U])(a: c.Expr[Append.Value[T, U]]): c.Expr[Setting[T]] =
     {
       val init = SettingMacro.settingMacroImpl[U](c)(v)
-      val assign = appendMacroImpl(c)(init.tree, a.tree)(Append1InitName)
-      c.Expr[Setting[T]](assign)
+      val append = appendMacroImpl(c)(init.tree, a.tree)(Append1InitName)
+      c.Expr[Setting[T]](append)
     }
   /** Implementation of ++= macro for tasks. */
   def taskAppendNImpl[T: c.WeakTypeTag, U: c.WeakTypeTag](c: Context)(vs: c.Expr[U])(a: c.Expr[Append.Values[T, U]]): c.Expr[Setting[Task[T]]] =
     {
       val init = taskMacroImpl[U](c)(vs)
-      val assign = appendMacroImpl(c)(init.tree, a.tree)(AppendNInitName)
-      c.Expr[Setting[Task[T]]](assign)
+      val append = appendMacroImpl(c)(init.tree, a.tree)(AppendNInitName)
+      c.Expr[Setting[Task[T]]](append)
     }
   /** Implementation of ++= macro for settings. */
   def settingAppendNImpl[T: c.WeakTypeTag, U: c.WeakTypeTag](c: Context)(vs: c.Expr[U])(a: c.Expr[Append.Values[T, U]]): c.Expr[Setting[T]] =
     {
       val init = SettingMacro.settingMacroImpl[U](c)(vs)
-      val assign = appendMacroImpl(c)(init.tree, a.tree)(AppendNInitName)
-      c.Expr[Setting[T]](assign)
+      val append = appendMacroImpl(c)(init.tree, a.tree)(AppendNInitName)
+      c.Expr[Setting[T]](append)
+    }
+
+  /** Implementation of -= macro for tasks. */
+  def taskRemove1Impl[T: c.WeakTypeTag, U: c.WeakTypeTag](c: Context)(v: c.Expr[U])(r: c.Expr[Remove.Value[T, U]]): c.Expr[Setting[Task[T]]] =
+    {
+      val init = taskMacroImpl[U](c)(v)
+      val remove = removeMacroImpl(c)(init.tree, r.tree)(Remove1InitName)
+      c.Expr[Setting[Task[T]]](remove)
+    }
+  /** Implementation of -= macro for settings. */
+  def settingRemove1Impl[T: c.WeakTypeTag, U: c.WeakTypeTag](c: Context)(v: c.Expr[U])(r: c.Expr[Remove.Value[T, U]]): c.Expr[Setting[T]] =
+    {
+      val init = SettingMacro.settingMacroImpl[U](c)(v)
+      val remove = removeMacroImpl(c)(init.tree, r.tree)(Remove1InitName)
+      c.Expr[Setting[T]](remove)
+    }
+  /** Implementation of --= macro for tasks. */
+  def taskRemoveNImpl[T: c.WeakTypeTag, U: c.WeakTypeTag](c: Context)(vs: c.Expr[U])(r: c.Expr[Remove.Values[T, U]]): c.Expr[Setting[Task[T]]] =
+    {
+      val init = taskMacroImpl[U](c)(vs)
+      val remove = removeMacroImpl(c)(init.tree, r.tree)(RemoveNInitName)
+      c.Expr[Setting[Task[T]]](remove)
+    }
+  /** Implementation of --= macro for settings. */
+  def settingRemoveNImpl[T: c.WeakTypeTag, U: c.WeakTypeTag](c: Context)(vs: c.Expr[U])(r: c.Expr[Remove.Values[T, U]]): c.Expr[Setting[T]] =
+    {
+      val init = SettingMacro.settingMacroImpl[U](c)(vs)
+      val remove = removeMacroImpl(c)(init.tree, r.tree)(RemoveNInitName)
+      c.Expr[Setting[T]](remove)
     }
 
   private[this] def appendMacroImpl(c: Context)(init: c.Tree, append: c.Tree)(newName: String): c.Tree =
@@ -170,6 +201,15 @@ object TaskMacro {
         case x => ContextUtil.unexpectedTree(x)
       }
     }
+  private[this] def removeMacroImpl(c: Context)(init: c.Tree, remove: c.Tree)(newName: String): c.Tree =
+    {
+      import c.universe.{ Apply, ApplyTag, newTermName, Select, SelectTag, TypeApply, TypeApplyTag }
+      c.macroApplication match {
+        case Apply(Apply(TypeApply(Select(preT, nmeT), targs), _), r) =>
+          Apply(Apply(TypeApply(Select(preT, newTermName(newName).encodedName), targs), init :: sourcePosition(c).tree :: Nil), r)
+        case x => ContextUtil.unexpectedTree(x)
+      }
+    }
   private[this] def transformMacroImpl(c: Context)(init: c.Tree)(newName: String): c.Tree =
     {
       import c.universe.{ Apply, ApplyTag, newTermName, Select, SelectTag }
diff --git a/notes/0.13.8.markdown b/notes/0.13.8.markdown
index 5386f3a79..4b31a0486 100644
--- a/notes/0.13.8.markdown
+++ b/notes/0.13.8.markdown
@@ -52,6 +52,7 @@
   [1881]: https://github.com/sbt/sbt/issues/1881
   [1899]: https://github.com/sbt/sbt/pull/1899
   [1902]: https://github.com/sbt/sbt/pull/1902
+  [1921]: https://github.com/sbt/sbt/issues/1921
 
 ### Changes with compatibility implications
 
@@ -135,6 +136,7 @@ This should help erase many of the deficiencies encountered when using Maven and
 - sbt-maven-resolver requires sbt 0.13.8 and above.
 - The current implementation does not support Ivy-style dynamic revisions, such as "2.10.+" or "latest.snapshot".  This
   is a fixable situation, but the version range query and Ivy -> Maven version range translation code has not been migrated.
+- The current implementation does not support Maven-style range revisions if found on transitive dependencies. [#1921][1921]
 
 [#1793][1793] by [@jsuereth][@jsuereth]
 
diff --git a/notes/0.13.9.markdown b/notes/0.13.9.markdown
new file mode 100644
index 000000000..9e2e4810a
--- /dev/null
+++ b/notes/0.13.9.markdown
@@ -0,0 +1,11 @@
+
+  [@dwijnand]: http://github.com/dwijnand
+  [1922]: https://github.com/sbt/sbt/pull/1922
+
+### Fixes with compatibility implications
+
+### Improvements
+
+- Add `-=` and `--=` for settings and tasks, dual of `+=` and `++=`. [#1922][1922] by [@dwijnand][@dwijnand]
+
+### Bug fixes
diff --git a/sbt/src/sbt-test/project/remove/build.sbt b/sbt/src/sbt-test/project/remove/build.sbt
new file mode 100644
index 000000000..0b967394c
--- /dev/null
+++ b/sbt/src/sbt-test/project/remove/build.sbt
@@ -0,0 +1,24 @@
+val intsTask = taskKey[Seq[Int]]("A seq of ints task")
+val intsSetting = settingKey[Seq[Int]]("A seq of ints setting")
+val intsFromScalaV = settingKey[Seq[Int]]("a seq of ints from scalaVersion")
+
+scalaVersion := "2.11.6"
+
+intsTask := Seq(1, 2, 3, 4, 5)
+intsTask -= 3
+intsTask --= Seq(1, 2)
+
+intsSetting := Seq(1, 2, 3, 4, 5)
+intsSetting -= 3
+intsSetting --= Seq(1, 2)
+
+intsFromScalaV := Seq(1, 2, 3, 4, 5)
+intsFromScalaV -= { if (scalaVersion.value == "2.11.6") 3 else 5 }
+intsFromScalaV --= { if (scalaVersion.value == "2.11.6") Seq(1, 2) else Seq(4) }
+
+val check = taskKey[Unit]("Runs the check")
+check := {
+  assert(intsTask.value == Seq(4, 5), s"intsTask should be Seq(4, 5) but is ${intsTask.value}")
+  assert(intsSetting.value == Seq(4, 5), s"intsSetting should be Seq(4, 5) but is ${intsSetting.value}")
+  assert(intsFromScalaV.value == Seq(4, 5), s"intsFromScalaV should be Seq(4, 5) but is ${intsFromScalaV.value}")
+}
diff --git a/sbt/src/sbt-test/project/remove/test b/sbt/src/sbt-test/project/remove/test
new file mode 100644
index 000000000..15675b169
--- /dev/null
+++ b/sbt/src/sbt-test/project/remove/test
@@ -0,0 +1 @@
+> check
diff --git a/src/main/conscript/sbt/launchconfig b/src/main/conscript/sbt/launchconfig
index 42fd0f075..f884aebcc 100644
--- a/src/main/conscript/sbt/launchconfig
+++ b/src/main/conscript/sbt/launchconfig
@@ -4,7 +4,7 @@
 [app]
   org: ${sbt.organization-org.scala-sbt}
   name: sbt
-  version: ${sbt.version-read(sbt.version)[0.13.7]}
+  version: ${sbt.version-read(sbt.version)[0.13.8]}
   class: sbt.xMain
   components: xsbti,extra
   cross-versioned: ${sbt.cross.versioned-false}
diff --git a/src/main/conscript/scalas/launchconfig b/src/main/conscript/scalas/launchconfig
index 49bfed9d2..b3a9cc26d 100644
--- a/src/main/conscript/scalas/launchconfig
+++ b/src/main/conscript/scalas/launchconfig
@@ -4,7 +4,7 @@
 [app]
   org: ${sbt.organization-org.scala-sbt}
   name: sbt
-  version: ${sbt.version-read(sbt.version)[0.13.7]}
+  version: ${sbt.version-read(sbt.version)[0.13.8]}
   class: sbt.ScriptMain
   components: xsbti,extra
   cross-versioned: ${sbt.cross.versioned-false}
diff --git a/src/main/conscript/screpl/launchconfig b/src/main/conscript/screpl/launchconfig
index 704c5aa51..c73788eed 100644
--- a/src/main/conscript/screpl/launchconfig
+++ b/src/main/conscript/screpl/launchconfig
@@ -4,7 +4,7 @@
 [app]
   org: ${sbt.organization-org.scala-sbt}
   name: sbt
-  version: ${sbt.version-read(sbt.version)[0.13.7]}
+  version: ${sbt.version-read(sbt.version)[0.13.8]}
   class: sbt.ConsoleMain
   components: xsbti,extra
   cross-versioned: ${sbt.cross.versioned-false}