#interest  a fixed charge for borrowing money; usually a percentage of the amount borrowed; "how much interest do you pay on your mortgage?"
  supertype:  #fixed_charge__fixedcharge  a periodic charge that does not vary with business volume (as insurance or rent or mortgage payments etc.)
  subtype:  #compound_interest  interest calculated on both the principal and the accrued interest
  subtype:  #simple_interest__simpleinterest  interest paid on the principle alone

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