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Tests for a decimal digit character
 
Integer_Type isdigit (String_Type s) 
This function returns a non-zero value if the first character in the
string s is a digit; otherwise, it returns zero.
 
A simple, user defined implementation of isdigit is
 
    define isdigit (s)
    {
       return ((s[0] <= '9') and (s[0]  >= '0'));
    }
However, the intrinsic function isdigit executes many times faster
than the equivalent representation defined above.
 
Unlike the C function with the same name, the S-Lang function takes
a string argument.
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