it is possible that the formular constructed by satchmo
contains variables that do not occur in the CNF sent to the solver

(e.g. if the variable never occurs in a constraint:
note that some constraints may be optimized away 
because they are always satisfied since they contain True.)

still these variables need an assignment
(we can choose any value). This should be done in 
package satchmo, module Satchmo.Solve