Fix choose not returning a stable type, add constructor to help

src/main/scala/leon/purescala/Constructors.scala

@@ -24,4 +24,12 @@ object Constructors {
     case xs =>
       LetTuple(xs, value, body)
   }
+
+  def tupleChoose(ch: Choose): Expr = {
+    if (ch.vars.size > 1) {
+      ch
+    } else {
+      Tuple(Seq(ch))
+    }
+  }
 }

src/main/scala/leon/synthesis/ConvertHoles.scala

@@ -63,7 +63,7 @@ object ConvertHoles extends LeonPhase[Program, Program] {
               BooleanLiteral(true)
           }
 
-          val withChoose = letTuple(holes, Choose(cids, pred), newBody)
+          val withChoose = letTuple(holes, tupleChoose(Choose(cids, pred)), newBody)
 
           fd.body = Some(withChoose)
         }

src/main/scala/leon/synthesis/ConvertWithOracles.scala

@@ -54,7 +54,7 @@ object ConvertWithOracle extends LeonPhase[Program, Program] {
                     BooleanLiteral(true)
                 }
 
-                Some(letTuple(os, Choose(chooseOs, pred), b))
+                Some(letTuple(os, tupleChoose(Choose(chooseOs, pred)), b))
               case None =>
                 None
             }

src/test/resources/regression/synthesis/Holes/Hole1.scala

@@ -3,26 +3,6 @@ import leon.collection._
 import leon.lang.synthesis._
 
 object Holes {
-  def abs1(a: Int) = {
-    if (?(true, false)) {
-      a
-    } else {
-      -a
-    }
-  } ensuring {
-    _ >= 0
-  }
-
-  def abs2(a: Int) = {
-    if (???[Boolean]) {
-      a
-    } else {
-      -a
-    }
-  } ensuring {
-    _ >= 0
-  }
-
   def abs3(a: Int) = {
     if (?(a > 0, a < 0)) {
       a