function ArcTan (x: Real): Real;
or
function ArcTan (z: Complex): Complex;
ArcTan returns the (principal value of the) arcus tangent of the argument. The result is in the range -Pi / 2 < ArcTan (x) < Pi / 2 for real arguments.
ArcTan is defined in ISO 7185 Pascal; its application to complex values is defined in ISO 10206 Extended Pascal.
program ArcTanDemo; begin { yields 3.14159 as ArcTan (1) = Pi / 4 } WriteLn (4 * ArcTan (1) : 0 : 5) end.