Backtracking and Constraints in Tcl-BC

Tcl is extended to include backtracking controls and a facility for asserting constraints. This increases its flexibility in several domains and opens the possibility for its use in constraint-based logic programming applications and, conversely, applying constraint-based logic programming techniques to more traditional script language domains. The backtracking extensions are written entirely in Tcl itself, a fact which speaks well for Tcl's own extensibility.