▁they ▁would ▁in ▁a ▁standard ▁inf ix ▁language . ▁ ▁The ▁calcul ator ▁man ages ▁this ▁by ▁allowing ▁the ▁implementation ▁of ▁these ▁blocks ▁to ▁skip ▁ahead ▁in ▁the ▁program ▁stream ▁as ▁necessary . ▁ ▁Cond itional ▁statements ▁ ▁IF / TH EN / EL SE / END ▁R PL ▁supports ▁basic ▁conditional ▁testing ▁through ▁the ▁IF / TH EN / EL SE ▁structure . ▁ ▁The ▁basic ▁syntax ▁of ▁this ▁block ▁is : ▁▁▁ ▁IF ▁condition ▁THEN ▁if - true ▁[ EL SE ▁if - false ] ▁END ▁ ▁The ▁following ▁example ▁tests ▁to ▁see ▁if ▁the ▁number ▁at ▁the ▁bottom ▁of ▁the ▁stack ▁is ▁" 1 " ▁and , ▁if ▁so , ▁rep laces ▁it ▁with ▁" Equal ▁to ▁one ": ▁▁▁ ▁« ▁IF ▁ 1 ▁== ▁THEN ▁" Equal ▁to ▁one " ▁END ▁» ▁ ▁The ▁IF ▁construct ▁evalu ates ▁the ▁condition ▁then ▁tests ▁the ▁bottom ▁of ▁the ▁stack ▁for ▁the ▁result . ▁ ▁As ▁a ▁result ▁R PL ▁can ▁option ally ▁support ▁FOR TH - style ▁IF ▁blocks , ▁allowing ▁the ▁condition ▁to ▁be ▁determined ▁before ▁the ▁block . ▁ ▁By ▁leaving ▁the ▁condition ▁empty , ▁the ▁IF ▁statement ▁will ▁not ▁make ▁any ▁changes ▁to ▁the ▁stack ▁during ▁the ▁condition ▁execution ▁and ▁will ▁use ▁the ▁existing ▁result ▁at ▁the ▁bottom ▁of ▁the ▁stack ▁for ▁the ▁test : ▁▁▁ ▁« ▁ 1 ▁== ▁IF ▁THEN ▁" Equal ▁to ▁one " ▁END ▁» ▁ ▁I FT / IF TE ▁Post fix ▁conditional ▁testing ▁may ▁be ▁accomplished ▁by ▁using ▁the ▁I FT ▁("