sumo#nonnegative_real_number  real_number >= 0
  closed_exclusion:  negative_real_number
  supertype:  real_number  a number that can be expressed as a (possibly infinite) decimal, i.e. a number that has a position on the number line
  subtype:  nonnegative_integer__nonnegativeinteger  integer >= 0
     subtype:  positive_integer  integer > 0
     subtype:  zero  class containing only 0
  subtype:  positive_real_number  real_number > 0
     subtype:  positive_integer  integer > 0

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