Notation of physical constants

If you want to use a physical constants inside Xcas, put it's name between two characters _ ("underscore"). Don't confuse physical constants with symbolic constants, for example, e,$\pi$ are symbolic constants as _c_,_NA_ are physical constants.
Input :

_c_

Output speed of light in vacuum :

299792458_m*s^-1

Input :

_NA_

Output Avogadro's number :

6.0221367e23_gmol^-1