anchor.diff:  external num pow(num x, num exponent);
   
   /**
  - * Converts [x] to a double and returns the sine of the value.
  - *
  - * If [x] is not a finite number, the result is NaN.
  - */
  -external double sin(num x);
  -
  -/**
  - * Converts [x] to a double and returns the cosine of the value.
  - *
  - * If [x] is not a finite number, the result is NaN.
  - */
  -external double cos(num x);
  -
  -/**
  - * Converts [x] to a double and returns the tangent of the value.
  - *
  - * The tangent function is equivalent to `sin(x)/cos(x)` and may be
  - * infinite (positive or negative) when `cos(x)` is equal to zero.
  - * If [x] is not a finite number, the result is NaN.
  - */
  -external double tan(num x);
  -
  -/**
  - * Converts [x] to a double and returns the arc cosine of the value.
  + * Converts [radians] to a [double] and returns the sine of the value.
  + *
  + * If [radians] is not a finite number, the result is NaN.
  + */
  +external double sin(num radians);
  +
  +/**
  + * Converts [radians] to a [double] and returns the cosine of the value.
  + *
  + * If [radians] is not a finite number, the result is NaN.
  + */
  +external double cos(num radians);
  +
  +/**
  + * Converts [radians] to a [double] and returns the tangent of the value.
  + *
  + * The tangent function is equivalent to `sin(radians)/cos(radians)` and may be
  + * infinite (positive or negative) when `cos(radians)` is equal to zero.
  + * If [radians] is not a finite number, the result is NaN.
  + */
  +external double tan(num radians);
  +
  +/**
  + * Converts [x] to a [double] and returns its arc cosine in radians.
    *
    * Returns a value in the range 0..PI, or NaN if [x] is outside
    * the range -1..1.
  
positive.code: 
  /**
   * Natural logarithm of 10.
   */
  const double LN10 = 2.302585092994046;
  
  /**
   * Natural logarithm of 2.
   */
  const double LN2 = 0.6931471805599453;
  
  
negative.code: external double acos(num x);
  
  /**
   * Converts [x] to a [double] and returns its arc sine in radians.
   *
   * Returns a value in the range -PI/2..PI/2, or NaN if [x] is outside
   * the range -1..1.
  