A formal logic *(term). See _("the Stanford Encyclopedia of Philosophy's entry" |http://plato.stanford.edu/entries/epsilon-calculus/).

In classical logic, an epsilon term <tt>epsilon(x) A</tt> denotes some value <var>x0</var> that satisfies <tt>A(x0)</tt> if and only if <tt>Exists(x) A(x)</tt>. Note that this is different from <em>ThereExists</em> in that it answers a satisfactory object (a "witness") instead of just the boolean answer to the query.

In some constructive logic, the epsilon term will converge to a witness if and only if a witness exists, and otherwise will diverge (fail to yield a value).

In a deep sense, epsilon is to <em>ThereExists</em> what lambda is to <em>ForAll</em>.

