finally, clean works :)

project/build/funcheck.scala

 import sbt._
 
-class FunCheckProject(info: ProjectInfo) extends ParentProject(info) with FileTasks {
+class FunCheckProject(info: ProjectInfo) extends DefaultProject(info) with FileTasks {
   override def outputDirectoryName = "bin"
   override def dependencyPath      = "lib"
   override def shouldCheckOutputDirectories = false
@@ -11,35 +11,50 @@ class FunCheckProject(info: ProjectInfo) extends ParentProject(info) with FileTa
 
   val scriptPath: Path = "." / "scalac-funcheck"
 
-  lazy val all = fileTask(scriptPath :: Nil)(allActions) dependsOn(plugin.`package`) describedAs("Compile everything and produce a script file.")
-
-  def allActions: Option[String] = {
-    val nl = System.getProperty("line.separator")
-    val f = scriptPath.asFile
-    val fw = new java.io.FileWriter(f)
-    fw.write("#!/bin/sh" + nl)
-    fw.write("LD_LIBRARY_PATH=" + ("." / "lib-bin").absolutePath + " \\" + nl)
-    fw.write("java \\" + nl)
-    val libStr = (buildLibraryJar.absolutePath).toString
-    fw.write("    -Dscala.home=" + libStr.substring(0, libStr.length-21) + " \\" + nl)
-    fw.write("    -classpath \\" + nl)
-    fw.write("    " + buildLibraryJar.absolutePath + ":")
-    fw.write(buildCompilerJar.absolutePath + ":")
-    fw.write(purescala.jarPath.absolutePath + ":")
-    fw.write(plugin.jarPath.absolutePath + ":")
-    fw.write(("lib" / "z3.jar").absolutePath + " \\" + nl)
-    fw.write("  scala.tools.nsc.Main -Xplugin:" + plugin.jarPath.absolutePath + " $@" + nl)
-    fw.close
-    f.setExecutable(true)
+  lazy val all = fileTask(scriptPath :: Nil)(generateScript) dependsOn(purescala.`package`, plugin.`package`) describedAs("Compile everything and produce a script file.")
+
+  override def cleanAction = super.cleanAction dependsOn(deleteScript)
+
+  lazy val deletescr = deleteScript
+
+  def deleteScript = task {
+    scriptPath.asFile.delete
     None
   }
 
+  def generateScript: Option[String] = {
+    try {
+      val nl = System.getProperty("line.separator")
+      val f = scriptPath.asFile
+      val fw = new java.io.FileWriter(f)
+      fw.write("#!/bin/sh" + nl)
+      fw.write("LD_LIBRARY_PATH=" + ("." / "lib-bin").absolutePath + " \\" + nl)
+      fw.write("java \\" + nl)
+
+      // This is a hack :(
+      val libStr = (buildLibraryJar.absolutePath).toString
+      fw.write("    -Dscala.home=" + libStr.substring(0, libStr.length-21) + " \\" + nl)
+
+      fw.write("    -classpath \\" + nl)
+      fw.write("    " + buildLibraryJar.absolutePath + ":")
+      fw.write(buildCompilerJar.absolutePath + ":")
+      fw.write(purescala.jarPath.absolutePath + ":")
+      fw.write(plugin.jarPath.absolutePath + ":")
+      fw.write(("lib" / "z3.jar").absolutePath + " \\" + nl)
+      fw.write("  scala.tools.nsc.Main -Xplugin:" + plugin.jarPath.absolutePath + " $@" + nl)
+      fw.close
+      f.setExecutable(true)
+      None
+    } catch {
+      case e => Some("There was an error while generating the script file: " + e.getLocalizedMessage)
+    }
+  }
+
   sealed abstract class PersonalizedProject(info: ProjectInfo) extends DefaultProject(info) {
     override def dependencyPath = "lib"
     override def outputDirectoryName = "bin" 
     override def compileOptions = super.compileOptions ++ Seq(Unchecked)
   }  
-
   class PureScalaProject(info: ProjectInfo) extends PersonalizedProject(info) {
     override def outputPath = "bin" / "purescala"
     override def mainScalaSourcePath = "src" / "purescala"
@@ -50,8 +65,6 @@ class FunCheckProject(info: ProjectInfo) extends ParentProject(info) with FileTa
     override def mainScalaSourcePath = "src" / "funcheck"
     override def unmanagedClasspath = super.unmanagedClasspath +++ purescala.jarPath
     override def mainResourcesPath   = "resources" / "funcheck"
-
-
   }
   class MultisetsProject(info: ProjectInfo) extends PersonalizedProject(info) {
     override def outputPath = "bin" / "multisets"