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  Title:    Partial Arctangent and Tangent
 
  Syntax:   FPATAN
            FPTAN
 
  See also: FSIN
 
  Description:
 
     FPATAN finds the partial tangent by calculating Z = ARCTAN(Y / X)
     where X is taken from ST and Y from ST(1). On the 8087/287, Y and
     X must be in the range 0 <= Y < X < infinity. On the 80387/486,
     there is no restriction on X and Y. X is popped from the stack and
     Z replaces Y in ST.
 
     FPTAN finds the partial tangent by calculating Y / X = TAN(Z). Z
     is taken from ST. Z must be in the range 0 <= Z <= pi/4 on the
     8087/287. On the 80387/486, |Z| must be less than 2^63. The result
     is the ratio Y / X. Y replaces Z, and X is pushed into ST. Thus, Y
     is returned in ST(1) and X in ST.
                                    -♦-