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5.23.1 Operators in Conditionals
--------------------------------

In 'if', 'ie', and 'while' requests, in addition to the numeric
expressions described in *note Numeric Expressions::, several Boolean
operators are available; the members of this expanded class are termed
"conditional expressions".

'c GLYPH'
     True if GLYPH is available, where GLYPH is an ordinary character, a
     special character '\(XX' or '\[XXX]', '\N'XXX'', or has been
     defined by any of the 'char', 'fchar', 'fschar', or 'schar'
     requests.

'd NAME'
     True if a string, macro, diversion, or request called NAME exists.

'e'
     True if the current page is even-numbered.

'F FONT'
     True if FONT exists.  FONT is handled as if it were opened with the
     'ft' request (that is, font translation and styles are applied),
     without actually mounting it.

'm COLOR'
     True if COLOR is defined.

'n'
     True if the document is being processed in 'nroff' mode.  *Note
     troff and nroff Modes::.

'o'
     True if the current page is odd-numbered.

'r REGISTER'
     True if REGISTER exists.

'S STYLE'
     True if STYLE is available for the current font family.  Font
     translation is applied.

't'
     True if the document is being processed in 'troff' mode.  *Note
     troff and nroff Modes::.

'v'
     Always false.  This condition is recognized only for compatibility
     with certain other 'troff' implementations.(1)  (*note Operators in
     Conditionals-Footnote-1::)

   If the first argument to an 'if', 'ie', or 'while' request begins
with a non-alphanumeric character apart from '!' (see below); it
performs an output comparison test.  (2)  (*note Operators in
Conditionals-Footnote-2::)

''XXX'YYY''
     True if formatting the comparands XXX and YYY produces the same
     output commands.  The delimiter need not be a neutral apostrophe:
     the output comparison operator accepts the same delimiters as most
     escape sequences; see *note Delimiters::.  This "output comparison
     operator" formats XXX and YYY in separate environments; after the
     comparison, the resulting data are discarded.

          .ie "|"\fR|\fP" true
          .el false
              => true

     The resulting glyph properties, including font family, style, size,
     and slant, must match, but not necessarily the requests and/or
     escape sequences used to obtain them.  In the previous example, '|'
     and '\fR|\fP' result in '|' glyphs in the same typefaces at the
     same positions, so the comparands are equal.  If '.ft I' had been
     added before the '.ie', they would differ: the first '|' would
     produce an italic '|', not a roman one.  Motions must match in
     orientation and magnitude to within the applicable horizontal and
     vertical motion quanta of the device, after rounding.  '.if
     "\u\d"\v'0'"' is false even though both comparands result in zero
     net motion, because motions are not interpreted or optimized but
     sent as-is to the output.(3)  (*note Operators in
     Conditionals-Footnote-3::) On the other hand, '.if "\d"\v'0.5m'"'
     is true, because '\d' is defined as a downward motion of one-half
     em.(4)  (*note Operators in Conditionals-Footnote-4::)

     Surround the comparands with '\?' to avoid formatting them; this
     causes them to be compared character by character, as with string
     comparisons in other programming languages.

          .ie "\?|\?"\?\fR|\fP\?" true
          .el false
              => false

     Since comparands protected with '\?' are read in copy mode (*note
     Copy Mode::), they need not even be valid 'groff' syntax.  The
     escape character is still lexically recognized, however, and
     consumes the next character.

          .ds a \[
          .ds b \[
          .if '\?\*a\?'\?\*b\?' a and b equivalent
          .if '\?\\?'\?\\?' backslashes equivalent
              => a and b equivalent

   The above operators can't be combined with most others, but a leading
'!', not followed immediately by spaces or tabs, complements an
expression.

     .nr x 1
     .ie !r x register x is not defined
     .el      register x is defined
         => register x is defined

   Spaces and tabs are optional immediately after the 'c', 'd', 'F',
'm', 'r', and 'S' operators, but right after '!', they end the predicate
and the conditional evaluates true.(5)  (*note Operators in
Conditionals-Footnote-5::)

     .nr x 1
     .ie ! r x register x is not defined
     .el       register x is defined
         => r x register x is not defined

The unexpected 'r x' in the output is a clue that our conditional was
not interpreted as we planned, but matters may not always be so obvious.