Definition 28: Given variables a, b, ..., z, and as many variables A, B, ..., Z , as well as as many variables \alpha, \beta, ..., \omega which are each distinct from all the variables mentioned so far, we define the statement [\phi | a \to A, b \to B, ..., z \to Z]_{\alpha, \beta, ..., \omega } to be given by the simple multiple variable substitution \big[ [\phi | a \to \alpha, b \to \beta, ..., z \to \omega]_\textup{simple} \big| \alpha \to A, \beta \to B, ..., \omega \to Z \big]_\textup{simple}. We refer to this as multiple variable substitution. If the context is clear, we may drop the subscript \alpha, \beta, ..., \omega and simply denote this by [\phi|a \to A, b \to B, ..., z \to Z].