A property used in conjunction with repeated_element_number to indicate an axis and directionality along that axis. If P preceding_element_is R, and P is_count_of S, and X P N, and X' P N+1, then it follows that every X R some X', and the class expression [S and R some X' and inv(R) some X] is empty (i.e. X is followed by X', with no intermediates)