**The Problem:** In `NormGrammarReader.processSymbol`, a cache is
maintained for symbols that are already processed. However, this cache
did not take into account that Symbols with and without a Label should
be treated as equal (e.g. `tail:Stm` and `Stm`, see
org.spoofax.jsglr2.integration/src/main/resources/grammars/layout-sensitive.sdf3).
These labels are only used to reference the symbols from the layout
constraints, but do not actually contribute to the meaning of a
production.

**Why is this a problem?** For SLR parse table generation (see metaborg#27), all
grammar symbols are assumed to be unique. However, because Symbols with
or without a Label were not treated as equal, the "graph" used to
calculate the First- and Follow-sets became disconnected. This in turn
caused some symbols to have empty First-/Follow-sets. The productions
belonging to these symbols would not get any reduce actions in the parse
table, because if the Follow-set is empty, no lookahead is allowed.

**Current solution:** When processing symbols in the `NormGrammarReader`,
any Label constructor is removed from the symbol term.

**Discussion:** I have tried looking around where these labels are used
(I am assuming layout constraints, but I couldn't find how they are
processed in NormGrammarReader).
Depending on their usage, it might be a good idea to apply this filter
in Stratego already, e.g. in the strategy `module-to-normal-form`.
@udesou any feedback is appreciated, as always 🙂