                     IF/ELSE/PROCEED STATEMENTS
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Every "if ... then" always has a unique corresponding "else"
                           and a unique corresponding "proceed"
 - clauses following "then" and "else" can have more than one statement
                           and may occupy more than one line
 - "if" statements can be nested; automatic prettyprinting shows levels

EXAMPLE:      if a=10 then if b=0 then write 'a=10, b=0' | q1=a*a
                  else write 'a=10, b<>0' | q1=0 | proceed
                else if b=0 then write 'a<>10, b=0'
                  else write 'a<>10,b<>0'
                  proceed
                proceed
              write 'done'

An "if" clause admits a single "and" OR a single "or" ONLY (no general
logic expression):

                if a=10 and b=0 then p=q else proceed

                (a=10) and (b=0) or q=2       IS ILLEGAL!
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