docs: public static BigDecimal atanh(BigDecimal x, MathContext mathContext) {
          if (x.compareTo(BigDecimal.ONE) >= 0) {
              throw new ArithmeticException("Illegal atanh(x) for x >= 1: x = " + x);
          }
          if (x.compareTo(MINUS_ONE) <= 0) {
              throw new ArithmeticException("Illegal atanh(x) for x <= -1: x = " + x);
          }
  
  		checkMathContext(mathContext);
  		MathContext mc = new MathContext(mathContext.getPrecision() + 6, mathContext.getRoundingMode());
  		BigDecimal result = log(ONE.add(x, mc).divide(ONE.subtract(x, mc), mc), mc).multiply(ONE_HALF, mc);
  		return round(result, mathContext);
  	}