omitting arguments in function/method calls

[Skaperen]

if you are just coding a call to some function referenced by a variable named "fun", and what is documented about it is that there are 3 arguments of related values, and you must omit one and it will calculate the missing one from the others and return that calculated value, the only way to do that is with named parameters?

actually, the function i am thinking of uses 4 values and 2 are to be omitted which it will calculate from the 2 that are given. i just asked about 3 to make it a simpler question. i was originally expecting a positional way to do it, like in another language it would be fun(a,,c).