As defined by the Federal Reserve Act, this term means "the legal gross indebtedness of the municipality (including the amount of any school district or other bonds which depend for their redemption upon taxes levied upon property within the municipality) less the aggregate of the following items:
"(1) The amount of outstanding bonds or other debt obligations made payable from current revenues;
"(2) The amount of outstanding bonds issued for the purpose of providing the inhabitants of a municipality with public utilities, such as waterworks, docks, electric plants, transportation facilities, etc.: Provided, That evidence is submitted showing that the income from such utilities is sufficient for maintenance, for payment of interest on such bonds, and for the accumulation of a sinking fund for their redemption;
(3) The amount of outstanding improvement bonds, issued under laws which provide for the levying of special assessments against abutting property in amounts sufficient to insure the payment of interest on the bonds and the redemption thereof: Provided, That such bonds are direct obligations of the municipality and included in the gross indebtedness of the municipality;
(4) The total of all sinking funds accumulated for the redemption of the gross indebtedness of the municipality, except sinking funds applicable to bonds just described in (1), (2), and (3) above."