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The notion of free crossed module was a major feature of JHC Whitehead's 1949 paoer "Combinatorial homotopy II", in which he proved, using methods of trwnsversality and knot theory, that the crossed module $$\pi_2(X \cup \{e^2_\lambda\},X,x) \to \pi_1(X,x) $$ is free on the characteristic maps of the $2$-cells. This theorem is sometimes mentioned but rarely proved in topology texts. Work of Philip Higgins and I showed how this theorem was a special case of a $2$-dim van Kampen type theorem, i.e. a colimit theorem, and the full story of this is given in Part I of the book partially titled Nonabelian Algebraic Topology, EMS 2011. Thus the more general theorem determines $$\pi_2(X \cup _f CA,A,x) \to \pi_1(X,x)$$ for $A$ connected, in terms of the induced morphism $f_*: \pi_1(A,a) \to \pi_1(X,x)$.