014 reject underscore outside branch handler

// TEST: `_` is legal ONLY as the (sole) body of a branch handler.
//
// The terminal `_` means "this branch handler intentionally does nothing";
// it has meaning only in `| branch [binding] |> _`. At the top level, or as
// a split-pipeline tail, there is no branch handler for `_` to be the body
// of — so `|> _` is meaningless and rejected.
//
// A void event is NOT special here: it chains monadically like anything
// else (`~hello() |> next()` is legal). What is rejected is the misplaced
// `_`, never "a redundant terminator after a void event".
//
// Legal:   ~hello()
// Legal:   ~hello() |> next()
// Illegal: ~hello() |> _

~tor hello {}

~proc hello|zig {}

~hello()
|> _