Diagrammatic visualization of the bias functions. The function φ₀(λ) is the generating function of all trees within one cell, φ₁(λ) of all trees within one cell with one external lines, and φ₂(λ) with two external lines. The resulting connected diagrams up to second order in ξ₁₂ are thus those presented here. Two φ₂ generating functions cannot be conntected as that would induce a loop contribution. This reflects the underlying tree structure.