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acos
Combined from primary sources listed below.

In Cool§

See primary documentation in context for routine acos. 

sub acos(Numeric(Cool))
method acos()

Coerces the invocant (or in sub form, the argument) to Numeric, and returns its arc-cosine in radians.

say 1.acos;                 # OUTPUT: «0␤»
say acos(-1);               # OUTPUT: «3.14159265358979␤»