#elisp
Function: isnan x

This predicate returns t if its floating-point argument is a NaN, nil otherwise.

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GNU Emacs Lisp Reference Manual: Float Basics
sp. Here are read syntaxes for these special floating-point values: infinity ‘ 1.0e+INF ’ and ‘ -1.0e+INF ’ not-a-number ‘ 0.0e+NaN ’ and ‘ -0.0e+NaN ’ The following functions are specialized for handling floating-point numbers: <span>Function: isnan x This predicate returns t if its floating-point argument is a NaN, nil otherwise. Function: frexp x This function returns a cons cell ( s . e ) , where s and e are respectively the significand and exponent of the floating-point number x . If x is finite