There are five inverters with drive strengths from <B>x2</B> to
<B>x12</B> and a P:N ratio of 1.4. There are also additional
layout variants <B>v6</B> and <B>v7</B>. The effort tables
show the effort relative to the reference inverter in the vsclib,
the <B>iv1v2x2</B>, which has a P:N ratio of 2.25.
