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acos(x [, degrees:T or radians:T or cycles:T]), x REAL or a structure
  with REAL components value in radians (default), cycles, or degrees as
  specified by option "angles" or the optional keyword

Keywords: transformations
acos(x) computes the inverse cosines of the elements of x, where x is a
REAL scalar, vector, matrix or array.  The result has the same shape as
x.  cos(acos(x)) should be the same as x except for rounding error.

The units of the result are radians, degrees or cycles as determined by
the value of option 'angles'.  The default is radians.  See subtopic

acos(x, radians:T), acos(x, degrees:T), acos(x, cycles:T) return results
in the indicated units, regardless of the value of option 'angles'.

When any element of x is MISSING or is above 1 or below -1, the
corresponding element of the result is MISSING and a warning message is

When x is a structure, all of whose non-structure components are REAL,
acos(x [,UNITS:T]), where UNITS is one of 'radians', 'degrees' or
'cycles', is a structure of the same shape and with the same component
names as x with each non-structure component transformed by acos().

  Cmd> vector(acos(.5),acos(.5,degrees:T),acos(.5,cycles:T))
  (1)      1.0472          60     0.16667

See topic 'transformations' for more information on acos(), including
its use with a CHARACTER argument.

Gary Oehlert 2003-01-15