The lexer returned a single ALTERNATION token for `?//`, so any `?`
followed by `//` was consumed by it, and `.name?//"unknown"` failed
to parse even though they read as the optional operator followed by
the alternative operator.
Drop the lexer rule and spell the destructuring alternative operator
as `'?' "//"` instead. So `x ?// y` now parses as `x? // y`, while
`. as [$a] ?// [$b] | ...` keeps its meaning. Note that `?` and `//`
may now be separated by whitespaces inside a pattern.
A4-Tacks
a19efdd45d