How do you implement SKBlendNode.alpha with an SKShader?

The available SKBlendNode values do not do what I want. I thought I'd try a custom SKShader. As a starting point, I'd like to modify SKBlendMode.alpha, which is described in the docs as:

The source and destination colors are blended by multiplying the source alpha value.


What does that mean in SKShader language?

void main() {
    vec4 src = texture2D(u_texture, v_tex_coord); // ??
    vec4 dest = ???
    gl_FragColor = ...;