function ArcSin (x: Real): Real;
or
function ArcSin (z: Complex): Complex;
ArcSin returns the (principal value of the) arcus sine of the argument. The result is in the range -Pi / 2 < ArcSin (x) < Pi / 2 for real arguments.
ArcSin is a GNU Pascal extension.
program ArcSinDemo; begin { yields 3.14159 as ArcSin (0.5) = Pi / 6 } WriteLn (6 * ArcSin (0.5) : 0 : 5) end.