Class Filter

Instruction --+    
              |    
          Stage --+
                  |
                 Filter

flow equivalent to filter:  Filter(function, stage, ... )

Yield those elements from a stage for which a function
returns true.   If the function is None, the identity 
function is assumed, that is, all items yielded that are
false (zero or empty) are discarded.

    def odd(val):
        if val % 2:
            return True
    
    def range():
        yield 1
        yield 2
        yield 3
        yield 4
    
    source = flow.Filter(odd,range)
    printFlow(source)