Update 'Phrases'

Phrases.md

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+ARF does not make a syntactical distinction between functions and operators. It
+instead has a concept called a phrase, which is similar to an [s-expression](https://en.wikipedia.org/wiki/S-expression) in lisp. A phrase is
+just a list of [arguments](Arguments), where the first argument is the function, method, or
+operator to execute, and the rest are, well, arguments to it.
+
+Phrases sometimes need to be delimited by square brackets:
+
+`[f arg1 arg2 arg3]`
+
+But, if the phrase does not contain any line breaks *directly* in it, and it is
+not being used as an argument, it can be written without the brackets:
+
+`f arg1 arg2 arg3`
+
+A phrase need not have brackets if the line break(s) inside of it are contained
+within brackets:
+
+```
+f [g
+	arg1
+	arg2] arg3
+```
+
+However, a phrase *must* have brackets if it is being used as an argument in any
+context:
+
+`f [g arg1 arg2] arg3`
+
+If the phrase has no brackets, it is terminated by the end of its line, or the presence of a [return direction](Return-Direction) arrow.