avoid duplicate residual goals for dif(X, Y) and dif(X, a)
avoid duplicate residual goals for dif(X, Y) and dif(X, a)

koryonik/swipl-devel,mndrix/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,jn7163/swipl-devel,mndrix/swipl-devel,mndrix/swipl-devel,mndrix/swipl-devel,jn7163/swipl-devel,jn7163/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel,edechter/swipl-devel