Sorts

sorts-list	:=	sort
	$\vert$	sort , sorts-list
sort	:=	sort-var
	$\vert$	sort -> sort
	$\vert$	( sort )
	$\vert$	alpha-ident
	$\vert$	alpha-ident [ sorts-list ]

Examples: prop -> prop, ('a -> 'b) -> list['a] -> list['b] are valid sorts.