-- Nested generic types: parenthesise compound types inside type ctors.
-- Lets a signature distinguish R-of-list from R-of-scalar precisely so
-- the verifier knows the Ok variant is a list, not an unrelated `L n` arg.
-- Validate a list, return Ok-wrapped list or Err message.
nz xs:L n>R (L n) t;l=len xs;=l 0 ^"empty";~xs
-- run: nz [1,2,3]
-- out: [1, 2, 3]
-- Value::Err from the entry function exits 1 with the err on stderr.
-- run: nz []
-- err: ^empty