+/* Returns a segmenter with the given syntax MODE.
+
+ If IS_SNIPPET is false, then the segmenter will parse as if it's being given
+ a whole file. This means, for example, that it will interpret - or + at the
+ beginning of the syntax as a separator between commands (since - or + at the
+ beginning of a line has this meaning).
+
+ If IS_SNIPPET is true, then the segmenter will parse as if it's being given
+ an isolated piece of syntax. This means that, for example, that it will
+ interpret - or + at the beginning of the syntax as an operator token or (if
+ followed by a digit) as part of a number.