@Section
   @Title { Defining new equation formatting symbols }
@Begin
@PP
Whenever you type particular equations or parts of equations 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
equations. @RawIndex { equations }
equations.definitions @SubIndex { definitions, use with }
definitions. @RawIndex { definitions }
definitions.use.with.equations @SubIndex { use with equations }
of their use in equation formatting.
@PP
Suppose for example that @OneCol @Eq { 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 "big sum from i=1 to n ` epi"
|7ct
@Eq { big sum from i=1 to n ` epi }
}
Parameters are very useful when parts of the symbol vary:
@ID @OneRow @Code {
"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 {
"big sum from i=1 to k ` ep i +"
"big sum from j=k+1 to n ep j"
}
||7ct
@Eq {
big sum from i=1 to k ` ep i +
big sum from j=k+1 to n ep j
}
}
The precedence of the symbols you define will be 100 by default.
@PP
To make the symbols of @Code "@Eq" available within such definitions,
each must be preceded by {@Code "import @Eq"}.  As explained in Section
{@NumberOf definitions}, the definitions go into a file called
{@Code "mydefs"}, which might then look like this:
@ID @OneRow @Code {
"import @Eq"
"def epi { p sub i ` log sub 2 ` p sub i }"
""
"import @Eq"
"def ep right x { p sub x ` log sub 2 ` p sub x }"
}
Use of @Code "epi" and @Code "ep" outside @Code "@Eq" will cause an error.
@End @Section