Pre-condition context should be used for POs inside a post-condition

[pglvdm]

For a CML model like:

process Test =
begin

state 
  m : map nat to nat:= {|->}

operations

Op(a: nat) r : nat
frame rd m 
pre a in set dom m
post r = m(a)

@ Skip

end

one gets a PO called:

(a in set (dom m))

but this should be seen in the context where the pre-condition is satusfied so one should get:

(a in set (dom m)) => (a in set (dom m))

which is probably will be possible to prove :-)

[ldcouto]

Fixed.

As with #223 , there is still an issue with collecting the state at the entry point of an operation. But the pre-condition is now correctly taken into account.