def, exist - functions to check whether a variable is defined or exists

Usage: def(<variable>)
       exist(<variable>)

def and exist are logical functions that take as argument a variable
name and return true if the variable exists (exist) or if it exists
and has value that differs from NULL (def) and false otherwise.

Examples: set x=NULL
          if (exist(x)) print "The variable x exists."
          The variable x exists.
          if (.not.def(x)) print "The variable x is undefined."
          The variable x is undefined.

See also: if, variable