wave addition. This allows you to add two arbitrary sine waves. Each wave is specified by an expression like A*sin(L*(x-f*t)) where A is the amplitude, L is the inverse of the wavelength, and f is the frequency. After you change the preset values (A = 2.5 etc), click on "change" to make that change take effect. To shift a wave by 180°, change the amplitude to its negative. To change phase by 90°, change the sin into a cos.