Tag Instructions

Format of tag instructions

Format	Explanation
`tag_<type>_<number>`	create a tag <number> whose value is taken from <type>
`untag_<number>`	remove the closest-maching tag
`return_tag_<type>_<number>`	return `tag_<type>_<number>` from <type> to `:return`
`tagged_<number>`	push the closest-maching to `:exec`
`tagged_code_<number>`	push the cloesest-maching to `:code`
`tagged_when_<number>`	check `:boolean`. true → `tagged_<number>`; false → `no_op`

pseudo-instruction	explanation
`tag-instruction?`	test whether an instruction is a tag-instruction or not
`closest-association`	return the closest match
`handle-tag-instruction`	execute tag instructions listed above
`*-instruction-erc`	return a tag instruction