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Return_Values

   x                  The tangent of the argument.
   HUGE_VAL           x is a singular point ( . . . -3pi/2, -pi/2,
                      pi/2 . . . ).
   NaN                x is NaN; errno is set to EDOM.
   0                  x is Infinity; errno is set to EDOM.
   HUGE_VAL           Overflow occurred; errno is set to ERANGE.
   0                  Underflow occurred; errno is set to ERANGE.