(2) The bonds shall be signed by the president of the board, attested with the seal of the district and by the signature of the secretary of the board. They may be issued so as to mature serially in annual amounts so as to be approximately equal, principal and interest, and may be issued so as to include a sum sufficient to pay the first four years’ interest, or less, to accrue on the bonds. [Amended by 1969 c.694 §36; 1977 c.188 §8; 1983 c.740 §215; 1991 c.459 §425t; 1997 c.631 §490; 2001 c.215 §20]