#once #create message.nf.EQdelim $$.ENSeveral of the examples we did earlier had Greek lettersand other mathematical symbols in them.The way to get things like $pi$ and $sum$ and $int$ israther easy - you just spell out their names, like this:$pi$ is pi$sum$ is sum$int$ is intand so on.The main thing you must always remember about these namesis that when they appear in an equation, they must beseparated from surrounding symbols by blanks or tildes,OR THEY WILL NOT BE RECOGNIZED.For practice, modify "Example" so the symbol $partial$is replaced by $sum$ everywhere it appears.Type "ready" when you're done..pl 1#once #create Ref.PPThe symbol.EQsum.ENoften appears in lines like.EQ{ sum x } over { sum y } = y over x.EN.pl 1##once neqn Ref | nroff >X1  &#once neqn message | nroff -T$term#once #create Example.PPThe symbol.EQpartial.ENoften appears in lines like.EQ{ partial x } over { partial y } = y over x.EN.pl 1#userneqn Example | nroff >X2#cmp X1 X2#log#next2.1d 10