anchor.diff:  /**
    * A variant of [atan].
    *
  - * Converts both arguments to doubles.
  - *
  - * Returns the angle between the positive x-axis and the vector ([b],[a]).
  - * The result, in radians, is in the range -PI..PI.
  + * Converts both arguments to [double]s.
  + *
  + * Returns the angle in radians between the positive x-axis
  + * and the vector ([b],[a]).
  + * The result is in the range -PI..PI.
    *
    * If [b] is positive, this is the same as `atan(b/a)`.
    *
  
positive.code: external double exp(num x);
  
  /**
   * Converts [x] to a double and returns the natural logarithm of the value.
   *
   * Returns negative infinity if [x] is equal to zero.
   * Returns NaN if [x] is NaN or less than zero.
  
negative.code: external num pow(num x, num exponent);
  
  /**
   * 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.
  