Testing for an empty sequence:

empty? ( seq -- ? )

Testing indices:

bounds-check? ( n seq -- ? )

Testing if a sequence contains an object:

member? ( elt seq -- ? )

member-eq? ( elt seq -- ? )

Testing if a sequence contains a subsequence:

head? ( seq begin -- ? )

tail? ( seq end -- ? )

subseq? ( subseq seq -- ? )

subseq-of? ( seq subseq -- ? )