testbox = box.Box()
testbox.put('c', 42)

@testbox.pass_('c')
def fn(a, b, c):
    return a + b + c

fn(1, 2, 3)        # (1) _?
fn(1, 2)           # (2) _?

fn(a=1, b=2, c=3)  # (3) _?
fn(a=1, b=2)       # (4) _?