`multi` should be parsed as terminal

[darosior]

When parsing a multi from Script we treat it as a non-terminal fragment. It's a terminal so it should be treated as such.

In this function:

python-bip380/bip380/miniscript/parsing.py

Lines 255 to 260 in 7b79c47

	def parse_nonterm_3_elems(expr_list, idx):
	"""
	Try to parse a non-terminal node from *at least* three elements of
	{expr_list}, starting from {idx}.
	Return the new expression list on success, None if there was no match.
	"""

python-bip380/bip380/miniscript/parsing.py

Lines 289 to 319 in 7b79c47

	# FIXME: multi is a terminal!
	# Match against a multi.
	try:
	k = stack_item_to_int(expr_list[idx])
	except ScriptNumError:
	return
	if k is None:
	return
	# <k> (<key>)* <m> CHECKMULTISIG
	if k > len(expr_list[idx + 1 :]) - 2:
	return
	# Get the keys
	keys = []
	i = idx + 1
	while idx < len(expr_list) - 2:
	if not isinstance(expr_list[i], fragments.Pk):
	break
	keys.append(expr_list[i].pubkey)
	i += 1
	if expr_list[i + 1] == OP_CHECKMULTISIG:
	if k > len(keys):
	return
	try:
	m = stack_item_to_int(expr_list[i])
	except ScriptNumError:
	return
	if m is None or m != len(keys):
	return
	node = fragments.Multi(k, keys)
	expr_list[idx : i + 2] = [node]
	return expr_list