A variable is a non-empty string over an alphabet used in a logical calculus to represent an element of a domain of discourse. More exactly, if $L$ is a formal language, $U$ the domain of discourse, and $s\in L$ an interpretable string with the interpretation $I(s)$, then a variable is any substring of $s$ being a symbol representing an element of $U$ in the interpretation $I(s)$.
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