Function require

18-with-match/lispy/original/lispy.py:272–274

Signal a syntax error if predicate is false.

(x, predicate, msg="wrong length")

        return list(map(expand, x))            # (f arg...) => expand each

def require(x, predicate, msg="wrong length"):
    "Signal a syntax error if predicate is false."
    if not predicate: raise SyntaxError(to_string(x)+': '+msg)

_append, _cons, _let = map(Sym, "append cons let".split())