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+@Section
+   @Title { Defining new mathematical symbols }
+@Begin
+@PP
+Whenever you type a particular piece of mathematics repeatedly, you
+can save time by using definitions.  Definitions are the subject of
+Section {@NumberOf definitions}, so here we will just give a few examples
+mathematics.definitions @SubIndex { definitions, use with }
+definitions. @RawIndex { definitions }
+definitions.use.with.mathematics @SubIndex { use with mathematics }
+of their use in formatting mathematics.
+@PP
+Suppose for example that @OneCol @Math { p sub i ` log sub 2 ` p sub i }
+occurs frequently in your document.  Then
+@ID @Code "def epi { p sub i ` log sub 2 ` p sub i }"
+makes the symbol @Code "epi" stand for the object between the braces:
+@ID {
+@Code @Verbatim { sum from { i=1 } to { n } ` epi }
+|7ct
+@Math { sum from { i=1 } to { n } ` epi }
+}
+Parameters are very useful when parts of the symbol vary:
+@ID @OneRow @Code @Verbatim {
+def ep right x
+{
+   p sub x ` log sub 2 ` p sub x
+}
+}
+The parameter @Code x will be replaced by the object just to the right
+of {@Code "ep"}:
+@ID {
+@Code {
+"sum from { i=1 } to { k } ` ep i +"
+"sum from { j=k+1 } to { n } ep j"
+}
+||7ct
+@Math {
+sum from { i=1 } to { k } ` ep i +
+sum from { j=k+1 } to { n } ep j
+}
+}
+The precedence of the symbols you define will be 100 by default.
+To make the symbols of @Code "@Math" available within such definitions,
+each must be preceded by {@Code "import @Math"}.  As explained in Section
+{@NumberOf definitions}, the definitions go into a file called
+{@Code "mydefs"}, which might look like this:
+@ID @OneRow @Code @Verbatim {
+import @Math
+def epi { p sub i ` log sub 2 ` p sub i }
+
+import @Math
+def ep right x { p sub x ` log sub 2 ` p sub x }
+}
+Use of @Code "epi" and @Code "ep" outside @Code "@Math" will cause an error.
+@End @Section