Push

The latest schush.ss includes a significant performance improvement and a new option for crossover mate selection:

http://hampshire.edu/lspector/schush.ss

20100121: - Eliminated calls to eval in Push program execution, speeding up all Push
            program execution significantly. This is done with an instruction name/procedure
            hash table to which instructions are added when registered. Note that now 
            ALL instructions that appear in Push programs must be defined with
            define-registered, rather than define.
          - Fixed examples in this file to define input instructions with define-registered.
          - Added compensatory mate selection, in which mates for crossover with individual 
            i are selected on the basis of low sums, over fitness cases, of the product
            of i's error and the crossover candidate's error. This is off by default but
            can be turned on by passing a value of #t to the compensatory-mate-selection
            argument of schushgp.