The scalar class #
Every closed fragment's class in the arity-zero Hom space is its
parameter value times the class of the empty fragment: this is
how the parameter f enters the skein category numerically.
The proof is the rank-one argument of Lemma 3.2 read backwards:
both rows of the arity-zero pairing are multiples of the empty
row, with ratio f(W).
The arity-zero pairing is the union value.
The scalar class: the class of a closed fragment in the arity-zero Hom space is its value times the empty class.