`digImplHoles` performs parsing twice

[scmu]

The function digImplHoles in Server.Handler.GCL.Refine parses implText twice.

• The purpose of the first parse is to, in case of parse errors, generate error messages with the correct column/line number. If the parse succeeds, the parse tree is simply thrown away.

• digImplHoles then parses implText again, with position 1 1 0, to "dig holes".

We can discuss whether it is a waste parsing twice, or it is actually an legitimate approach that works well in practice.