is an attribute that can be assigned to a symbol f to indicate that all expressions involving nested functions f should be flattened out. This property is accounted for in pattern matching.


  • Flat corresponds to the mathematical property of associativity.
  • For a symbol f with attribute Flat, is automatically reduced to .
  • Functions like Plus, Times, and Dot are Flat.
  • For a Flat function f, the variables x and y in the pattern can correspond to any sequence of arguments.
  • The Flat attribute must be assigned before defining any values for a Flat function.
  • When functions that are Flat are used in pattern matching, they often also require the attribute OneIdentity.
Introduced in 1988
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