Reduce Compact Letter Displays

Reduces compact letter displays (CLDs) of all pairwise comparisons. A CLD gives two groups a common letter exactly when they are not significantly different. The package finds a display with the fewest letter assignments that keeps every one of these relationships. This is the assignment-minimum clique covering problem of Ennis, Fayle, and Ennis (2012) , solved as a mixed-integer linear program with 'HiGHS'. Among the displays with the fewest assignments, it returns one fixed canonical display, so the result does not depend on the solver. The package includes the wheat yield example of Piepho (2004) .