### Compilation failed:

error: 5: 'main' must return: 'vec4', 'float4', or 'half4'
void main() {
^^^^^^^^^^^
error: 6: unknown identifier 'sk_FragColor'
    sk_FragColor.x = cross_length_2d(ah, bh);
    ^^^^^^^^^^^^
error: 6: unknown identifier 'cross_length_2d'
    sk_FragColor.x = cross_length_2d(ah, bh);
                     ^^^^^^^^^^^^^^^
error: 7: unknown identifier 'sk_FragColor'
    sk_FragColor.y = half(cross_length_2d(af, bf));
    ^^^^^^^^^^^^
error: 7: unknown identifier 'cross_length_2d'
    sk_FragColor.y = half(cross_length_2d(af, bf));
                          ^^^^^^^^^^^^^^^
5 errors