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Amalgamation Property

An amalgam is a tuple $\langle \mathbf{A},f,\mathbf{B},g,\mathbf{C}\rangle$ such that $\mathbf{A},\mathbf{B},\mathbf{C}$ are structures of the same signature, and $f:\mathbf{A}\to\mathbf{B}$, $g:\mathbf{A}\to\mathbf{C}$ are embeddings (injective morphisms).

A class $\mathcal{K}$ of structures is said to have the amalgamation property if for every amalgam with $\mathbf{A},\mathbf{B},\mathbf{C}\in\mathcal{K}$ and $A\neq\varnothing$ there exists a structure $\mathbf{D}\in\mathcal{K}$ and embeddings $f ':\mathbf{B}\to\mathbf{D}$, $g':\mathbf{C}\to\mathbf{D}$ such that $f '\circ f=g'\circ g$.