Implement full flexibility in transformer arguments

[ReedOei]

It is sometimes useful to allow the arguments in transformer flows to be in positions other than the final position. We should allow this using this following syntax, using an underscore to indicate the actual position of the argument. This should be required once implemented, because x --> f() --> y looks weird. We should forbid _ as a variable name.

Src --> F(x1, ..., xi, _, xj, ..., xn) --> Dst

Additionally, this same syntax should be used in filters, so we can (/have to) now write:

Src --[ q such that P(x1, ..., xi, _, xj, ..., xn) ]-> Dst