Modula-3: If statements

## If

An `IF` statement has the form:

        IF    B_1 THEN S_1
        ELSIF B_2 THEN S_2
          ...
        ELSIF B_n THEN S_n
        ELSE S_0
        END

where the `B`'s are boolean expressions and the `S`'s are statements. The "`ELSE S_0`" and each "`ELSIF B_i THEN S_i`" are optional.

The statement evaluates the `B`'s in order until some `B_i` evaluates to `TRUE`, and then executes `S_i`. If none of the expressions evaluates to `TRUE` and "`ELSE S_0`" is present, `S_0` is executed. If none of the expressions evaluates to `TRUE` and "`ELSE S_0`" is absent, the statement is a no-op (except for any side-effects of the `B`'s).

    Last modified on Wed Apr  3 09:39:07 PST 1996 by heydon
         modified on Mon Apr 18 14:03:35 PDT 1994 by kalsow