Editing Unification (computer science) (section)

===Substitution===
{{main|Substitution (logic)}}
A ''substitution'' is a mapping <math>\sigma: V\rightarrow T</math> from variables to terms; the notation <math> \{x_1\mapsto t_1, ..., x_k \mapsto t_k\}</math> refers to a substitution mapping each variable <math>x_i</math> to the term <math>t_i</math>, for <math>i=1,...,k</math>, and every other variable to itself; the <math>x_i</math> must be pairwise distinct. ''Applying'' that substitution to a term <math>t</math> is written in [[postfix notation]] as <math>t \{x_1 \mapsto t_1, ..., x_k \mapsto t_k\}</math>; it means to (simultaneously) replace every occurrence of each variable <math>x_i</math> in the term <math>t</math> by <math>t_i</math>. The result <math>t\tau</math> of applying a substitution <math>\tau</math> to a term <math>t</math> is called an ''instance'' of that term <math>t</math>.
As a first-order example, applying the substitution {{math|{{mset| ''x'' ↦ ''h''(''a'',''y''), ''z'' ↦ ''b'' }}}} to the term 
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| align="center" | <math>\textbf{x}</math>
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| yields &nbsp;
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