Man pages matching name identity_matrix in section 3alleg4:

identity_matrix.3alleg4.en	debian 10 / allegro4-doc
identity_matrix.3alleg4.en	debian 11 / allegro4-doc
identity_matrix.3alleg4.en	debian 12 / allegro4-doc
identity_matrix.3alleg4.en	debian 13 / allegro4-doc