What: generator - Generators for Tcl
Where: http://www.cs.nott.ac.uk/~nem/tcl/ 
       http://www.cs.nott.ac.uk/~nem/tcl/generator.html 
Description: Generators are procedures that produce a series of values. 
     Rather than returning a list, a generator "yields" each value in turn and then suspends 
     until the next value is requested.  The generator package provides a number of 
     convenience routines for creating and using generators, building on top of the 
     co-routine mechanism in Tcl 8.6.