Type of a variable is the intended set from which the value may be taken
/ has type Num †¥ (Num-{0})ÆNum
T* means the type
"lists of things of type T"
range: Num †¥ Num Æ Num*
range(1,5) ==> [1,2,3,4,5]
Type of
reduce: (T †¥ TÆT) †¥ T †¥ T*ÆT
fn unit list
