One entry of the NumericFns registry: the parameters R declares, in order, and how to fold them.
A call supplies its arguments positionally or under these names, and fold receives them in declaration
order, so a parameter R gives a default is simply an optional parameter of fold. Whatever fold cannot
answer (a missing operand it needs, a domain error) it returns undefined for, which keeps the call Top.
One entry of the NumericFns registry: the parameters R declares, in order, and how to fold them.
A call supplies its arguments positionally or under these names, and
foldreceives them in declaration order, so a parameter R gives a default is simply an optional parameter offold. Whateverfoldcannot answer (a missing operand it needs, a domain error) it returnsundefinedfor, which keeps the callTop.