A power of appointment, whether or not coupled with an interest, and whether or not existing at the time this section takes effect, and whether the power is held by the donee in an individual or in a fiduciary capacity, may be released, wholly or partially, by the donee thereof, unless otherwise expressly provided in the instrument creating the power. As used in this chapter, the term power of appointment includes all powers which are in substance and effect powers of appointment regardless of the language used in creating them and whether they are: (a) general, special, or otherwise; (b) in gross, appendant, simply collateral, in trust, or otherwise; (c) exercisable by will, deed, deed or will, or instrument amending a trust, or otherwise; (d) exercisable presently or in the future.