Incomprehensible function signature - Return reference to an array of N objects

Question

I\'ve come across the following signature

double(&rotate_vec(double(&val)[4]))[4];

In the comments it \"claims\" to accept and

Answer 1

double ( &rotate_vec( double (&val)[4] ) )[4]

A function named rotate_vec

double ( &rotate_vec( double (&val)[4] ) )[4]

...that takes as an argument, a reference to an array of four doubles

double ( &rotate_vec( double (&val)[4] ) )[4]

...and returns a reference to an array of four doubles.