+@code{readline.h} defines a C preprocessor variable that should
+be treated as an integer, @code{RL_READLINE_VERSION}, which may
+be used to conditionally compile application code depending on
+the installed Readline version. The value is a hexadecimal
+encoding of the major and minor version numbers of the library,
+of the form 0x@var{MMmm}. @var{MM} is the two-digit major
+version number; @var{mm} is the two-digit minor version number.
+For Readline 4.2, for example, the value of
+@code{RL_READLINE_VERSION} would be @code{0x0402}.
+