Constructs a bag of the default implementation 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.