Class Filter

Instruction --+        
              |        
          Stage --+    
                  |    
               Pipe --+
                      |
                     Filter

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

Yield those elements from a source 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)