The equations for higher-spin fields originate from the Vasiliev equations in the unfolded form. Any set of differential equations can be put in the first order form by introducing auxiliary fields to denote derivatives. Unfolded approach is an advanced reformulation of this idea that takes into account gauge symmetries and diffeomorphisms. Instead of just ∂ ∂ μ μ ϕ ϕ i ( x ) = f μ μ i ( ϕ ϕ ) the unfolded equations are written in the language of differential forms as