Marks a L0 top-level symbol as discarded
- IOW the marked symbol is treated as whitespace which is not passed to G1.
- Usage of the symbol in G1 is a fatal error.
- As is the symbol not being top-level (i.e. being used in some L0 RHS).
The clause can set an optional discard event for the symbol.
See also discard default for the global discard event settings.