Let's say you have one language that supports double-quoted strings and interpolates local variables, like this: x = 17 print "The number is $x" produ

Let's say you have one language that supports double-quoted strings and interpolates local variables, like this: x = 17 print "The number is $x" produces: The number is 17 Let's say in some situation you don't want interpolation, and so you want to mix in some other language that supports interpolation-less strings, and it just happens to use triple-quoted strings as the syntax for that behaviour. So, we make a new language by combining the original language's expression syntax with the new language's expressions syntax using the ordered choice operator. Then we feed it this input: x = 17 print """The number is $x""" This time, the first language's parser sees the opening quote, then immediately afterward it sees a closing quote. Then it sees another opening quote, some text, an interpolation and a closing quote, then lastly another opening and closing quote, so it yields three string literals and concatenates them to produce one large, interpolated string. Even though our combined grammar had a special rule for triple-quoted strings, we couldn't use it just by blindly concatenating parts of the grammar; we'd have to rewrite both of them a little bit to get them to mesh nicely. Granted, the above is a slightly contrived example, but most non-trivial PEG grammars have to pay at least some attention to the order of their ordered choice operands, even within a single language.