Recursive step
if n > m : NEW_LINE INDENT return Hosoya ( n - 1 , m ) NEW_LINE INDENT + Hosoya ( n - 2 , m ) NEW_LINE DEDENT DEDENT elif m == n : NEW_LINE INDENT return Hosoya ( n - 1 , m - 1 ) NEW_LINE INDENT + Hosoya ( n - 2 , m - 2 ) NEW_LINE DEDENT DEDENT else : NEW_LINE INDENT return 0 NEW_LINE DEDENT