> Why require that each "if-then" has an "else"

Never mind the semantics, this is a piece of syntactic history.

ALGOL 60 allowed omitted "else". That gave ambiguity:

    if p
       then if q 
               then ...
           ...
           else ...

There was no layout rule. Does the "else" match to the outer "if-then" or the inner?

BCPL ~1965 had two constructs

    if p
       then test q
                 then ...
           ...
           else ...

"if-then" has no "else". If you want an "else" use "test-then-else".


AntC