Start with x, y; then concatenate each word in turn with all preceding words, getting x y xy xxy yxy xxxy yxxy xyxxy ...; sequence gives number of words of length n. Also binary trees by degree: x y (x,y) (x,(x,y)) (y,(x,y)) (x,(x,(x,y))) (y,(x,(x,y))) ((x,y),(x,(x,y)))...

A063894

Start with x, y; then concatenate each word in turn with all preceding words, getting x y xy xxy yxy xxxy yxxy xyxxy ...; sequence gives number of words of length n. Also binary trees by degree: x y (x,y) (x,(x,y)) (y,(x,y)) (x,(x,(x,y))) (y,(x,(x,y))) ((x,y),(x,(x,y)))...