Lets work in the compactly generated topological spaces category. Let K be the Stasheff's operad (which algebras are $A_\infty$-spaces with strict unit). Please, I would like to understand how K can be given as the relative W-construction $W(I^* \to As)$, where $I^*$ is the operad of pointed spaces and As is the operad of unital monoids.
More precisely, I would like to know what are the specific identifications involving trees in $W(I^* \to As)$.