Definition:Substitution Instance

A substitution instance of a sequent $$S_1$$ is a sequent $$S_2$$ which results from $$S_1$$ by replacing one or more of the statement variables in some statement forms in $$S_1$$ throughout the whole of $$S_1$$ by other statement forms, as follows:


 * Every occurrence of a particular substituted statement variable is replaced by the same statement form;
 * Different statement variables may be replaced by different statement forms;
 * The substitution can be made only on statement variables.