Disabled nested quantification for BigInts in Predicate testcase

HOL

Isabelle can't prove termination for those methods which have been
introduced in 61c73029.

Fixed compilation issue in String.scala
Made sure (...).substring(start) is extracted as a lset in order to not recompute the body twice (and adapted the pretty-printer)

1) testCondition should refer to current problem, not top-level function

2) In match, we should not add values to the eb of the problem if we
don't add variables

3) When only input variables are involved, substitute them in the
problem

This reverts commit 4da35a7a.

Prefer shorter deletions.

Added faster version for abstract-only evaluation.
Fixed bug of tuples in AbstractEvaluator.scala

Bindings get bubbled as early in the sequence of bindings/conditions as possible

Define Element type

'true' conditions get filtered out

added pretty printing for termination checker

This reverts commit 8e6cfbae.

alternations

anyway)

Fixed ExprOps.isValue for covariant/contravariant function types.