  The existence of a median-type ternary operation on a metric space is known
to have a number of implications for the geometry of the space. For such
operations, if two of the three arguments coincide, they also coincide with the
output of the operation. We consider ternary operations with the opposite
property: if two of the arguments coincide, the output is equal to the third
one. The existence of such an operation is a necessary condition for the space
to be an absolute retract.
