add new syntax combining a for loop and a comprehension

Currently, if we want to combine a comprehension and a for loop in a single line, we must write something of the form:

@#for (C1,C2,S) in [ (C1,C2,S) in CTR^2*SEC when C1 != C2 ]

which is rather weird since the indices have to be repeated.

I suggest to add a new syntax that directly combines the two:

@#for (C1,C2,S) in CTR^2*SEC when C1 != C2