+% @ifcommandisdefined CMD ... @end executes the `...' if CMD (written
+% without the @) is in fact defined. We can only feasibly check at the
+% TeX level, so something like `mathcode' is going to considered
+% defined even though it is not a Texinfo command.
+%
+\makecond{ifcommanddefined}
+\def\ifcommanddefined{\parsearg{\doifcmddefined{\let\next=\ifcmddefinedfail}}}
+%
+\def\doifcmddefined#1#2{{%
+ \makevalueexpandable
+ \let\next=\empty
+ \expandafter\ifx\csname #2\endcsname\relax
+ #1% If not defined, \let\next as above.
+ \fi
+ \expandafter
+ }\next
+}
+\def\ifcmddefinedfail{\doignore{ifcommanddefined}}
+
+% @ifcommandnotdefined CMD ... handled similar to @ifclear above.
+\makecond{ifcommandnotdefined}
+\def\ifcommandnotdefined{%
+ \parsearg{\doifcmddefined{\else \let\next=\ifcmdnotdefinedfail}}}
+\def\ifcmdnotdefinedfail{\doignore{ifcommandnotdefined}}
+
+% Set the `txicommandconditionals' variable, so documents have a way to
+% test if the @ifcommand...defined conditionals are available.
+\set txicommandconditionals
+