text: var annotate = require('annotate');
  var is = require('annois');
  
  
  module.exports = annotate('acos', 'Returns the arccosine (in radians) of a number').
      on(function betweenOnes(a) {
          return is.number(a) && -1 <= a && a <= 1;
      }, Math.acos);
  