Constructs a wb-bag according to the supplied argument subforms. Each
argument subform can be an expression, whose value will be added to the bag
with multiplicity 1; or a list of the form ($ expression), in which case the
expression must evaluate to a bag (or a set), which is bag-summed into the
result; or a list of the form (% expression1 expression2) (called a
“multi-arg”), which indicates that the value of expression1 is bag-summed
into the result with multiplicity given by the value of expression2. That
is, the multiplicity of each member of the result bag is the sum of its
multiplicities as supplied by each of the argument subforms.