  We propose a construction of the monoidal envelope of $\infty$-operads in the
model of Segal dendroidal spaces, and use it to define cocartesian fibrations
of such. We achieve this by viewing the dendroidal category as a "plus
construction" of the category of pointed finite sets, and work in the more
general language of algebraic patterns for Segal conditions. Finally, we
rephrase Lurie's definition of cartesian structures as exhibiting the
categorical fibrations coming from envelopes, and deduce a
straightening/unstraightening equivalence for dendroidal spaces.
