No support for infinity in Rascal.
The "Rascal" way of dealing with such variability is to introduce an algebraic data-type, as in:
data Arity = inf() | fixed(int size)
Then you can use pattern matching or is or whatever to deal with the differences.
if (arity is inf) {...}
int foo(fixed(int size)) = ...;
int foo(inf()) = ...;