The dangling else problem of Pascal was not shared by its predecessor Algol 60. To avoid ambiguity regarding which then is matched by an else, Al-gol 60 prohibited if statements immediately inside a then clause. The Pascal fragment

if C1 then if C2 then S1 else S2

had to be written as either

if C1 then begin if C2 then S1 end else S2

or

if C1 then begin if C2 then S1 else S2 end

in Algol 60. Show how to write a grammar for conditional statements that enforces this rule.