Add missing implementation for ecInfFromThen, which models [a,b...]
Add missing implementation for ecInfFromThen, which models [a,b...]

Fixes GaloisInc/saw-script#22.

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script