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this allows to provide the time limit on the command of CVC4, and perhaps Z3 later.

Conflicts:
	examples/bts/execute.mlw

(reported by mario.alvarez739@gmail.com)

slight improvements of CVC4 driver, preparing the support for the "steps" feature and the counter-examples

1. We really need to distinguish first-class functions (mappings),
pure logical functions (functions and predicates), and program
functions (routines).

2. The ps_ prefix is already used for predicate symbols.

3. 'psymbol' and 'pvsymbol' are too similar.