QoL: if parser should accept known symbols for its case

Like

import flowrep as fr

def identity(x):
    return x

@fr.workflow
def macro(x, y, z):
    if x:
        w = identity(y)
    else:
        w = identity(z)
    return w

Currently you need an identity wrapper like if identity(x).

Under the hood, the recipe still expects an operation for the case, and I can tolerate that, I'd be content if it just auto-inserted the identity operation at parsing time so I can write more straightforward python.

Start by locating the parser used by the @fr.workflow decorator and the handling of if cases. Reproduce the example without the identity wrapper, then trace how the recipe represents the case operation. Done means the parser accepts known symbols directly while preserving the expected operation in the recipe; add or update parser tests if the project has them.