Deforming Paths of Integration for Triangular Contours

In this file, we prove that if a function satisfies the Cauchy-Goursat conditions for rectangular contours, then the integral along the diagonal of the rectangle is equal to the integral along both pairs of perpendicular sides of the rectangle.

theorem Complex.re_of_real_add_real_mul_I (x y : ℝ) : (↑x + ↑y * I).re = x

theorem Complex.im_of_real_add_real_mul_I (x y : ℝ) : (↑x + ↑y * I).im = y