Alternative wp interpretator with sampling instead of expectation using
'System.Random.randInt'.

tick/consume now takes an expression
tick e = max(0,e)

- parse invariants [bexp]
- add invariants for t30/t15
- add examples/couponcollector.imp (working)

diffs in guessNorms seems to be empty always

seems to work; not necessarily better bounds

Solver works with optimathsat; atm open whether optimisation used
correctly. Currently turned off.