Steps in the Representation of Concept Lattices and Median Graphs.

Median semilattices have been shown to be useful for dealing with phylogenetic classication problems since they subsume median graphs, distributive lattices as well as other tree based classica-tion structures. Median semilattices can be thought of as distributive ∨-semilattices that satisfy the following property (TRI): for every triple x, y, z, if x ∧ y, y ∧ z and x ∧ z exist, then x ∧ y ∧ z also exists. In previous work we provided an algorithm to embed a concept lattice L into a dis-tributive ∨-semilattice, regardless of (TRI). In this paper, we take (TRI) into account and we show that it is an invariant of our algorithmic approach. This leads to an extension of the original algorithm that runs in polynomial time while ensuring that the output is a median semilattice.