title: How to get a default EquivalenceLibrary?
body: <p>I want to use the BasisTranslator in Qiskit to process my circuit to base gates.</p>
  <p>One of the constructor parameters for <a href="https://qiskit.org/documentation/stubs/qiskit.transpiler.passes.BasisTranslator.html#qiskit.transpiler.passes.BasisTranslator" rel="nofollow noreferrer">BasisTranslator</a> is <a href="https://qiskit.org/documentation/stubs/qiskit.circuit.EquivalenceLibrary.html#qiskit.circuit.EquivalenceLibrary" rel="nofollow noreferrer">EquivalenceLibrary</a>. I attempted creating a parameter-less <code>EquivalenceLibrary()</code> and pass it to a BasisTranslator, but the code fails, saying it cannot translate gates like MCT or CCX into the default basis.</p>
  <pre><code>TranspilerError: &quot;Unable to map source basis {('mcx', 4), ('ccx', 3), ('x', 1), ('measure', 1), ('h', 1)} to target basis {'u2', 'snapshot', 'barrier', 'cx', 'u3', 'reset', 'u1', 'measure', 'delay'}
  </code></pre>
  <p>I would expect there is some default EquivalenceLibrary that the Transpiler uses when I attempt to run the circuit, since it runs both for simulators and actual hardware. How could I get such default EquivalenceLibrary?</p>