function ArcCos (x: Real): Real;
or
function ArcCos (z: Complex): Complex;
ArcCos returns the (principal value of the) arcus cosine of the argument. The result is in the range 0 < ArcCos (x) < Pi for real arguments.
ArcCos is a GNU Pascal extension.
program ArcCosDemo; begin { yields 3.14159 as ArcCos (0.5) = Pi / 3 } WriteLn (3 * ArcCos (0.5) : 0 : 5) end.