optional int parameters for a proc

[jlp765]

**Issue**:
    Need `int` type parameters to be optional, and to test whether the 
parameter has been passed to the `proc` or not, BUT the `int` type does not 
allow a `nil` value

So, how do you do the equivalent of
    
    
    proc xxx(param1: string, param2: int = nil, param3: int = nil)
      if not param2.isNil():
        # do something with param2