pd_sym(name)                 pre-defined symbol access

    Return the pre-defined  variable with name  equal to the  string of the
    function parameter, "name".

pd_sym(n,name)               pre-defined symbol access

    Return the pre-defined variable with position in the symbol table equal
    to the integer value of  n. Variable name returned in  variable "name".
    For this form "name" MUST be a variable.

    If the integer value of n  is zero, negative or greater than  number of
    pre-defined variables, the null string is returned.