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'use strict';
var log = Math.log;
module.exports = function (x) {
if (isNaN(x)) return NaN;
x = Number(x);
if (x < -1) return NaN;
if (x > 1) return NaN;
if (x === -1) return -Infinity;
if (x === 1) return Infinity;
if (x === 0) return x;
return 0.5 * log((1 + x) / (1 - x));
};
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