G.6.2.2 The casinh functions
casinh(conj(z)) = conj(casinh(z)) and casinh is odd.
casinh(+0 + i0) returns 0 + i0.
casinh(x + i (inf)) returns +(inf) + ipi /2 for positive-signed finite x.
casinh(x + iNaN) returns NaN + iNaN and optionally raises the “invalid” floating-point exception, for finite x.
casinh(+(inf) + iy) returns +(inf) + i0 for positive-signed finite y.
casinh(+(inf) + i (inf)) returns +(inf) + ipi /4.
casinh(+(inf) + iNaN) returns +(inf) + iNaN.
casinh(NaN + i0) returns NaN + i0.
casinh(NaN + iy) returns NaN + iNaN and optionally raises the “invalid” floating-point exception, for finite nonzero y.
casinh(NaN + i (inf)) returns (+-)(inf) + iNaN (where the sign of the real part of the result is unspecified).
casinh(NaN + iNaN) returns NaN + iNaN.