Introduce a second-order function
via a second-order version of define.
(define2 sofun ...) ; same as define
The inputs are identical to define.
The function sofun must satisfy
all the requirements for defsoft,
because define2 generates (defsoft sofun) (see below).
(define sofun ...) ; input form with define2 replaced by define
sofun is introduced as a first-order function using define.
It is also recorded as a second-order function via defsoft.
- Implementation of define2.