improve var creation for SumProdCircuit

examples/quotient.rs

@@ -19,6 +19,7 @@ fn main() {
     let quotient: Rc<RefCell<QuotientCircuit>> = idem.into();
 
     // Build x * x̄ + x in the DAG
+    // var accepts anything that implements Into<StaticVar>: &'static str, (&str, u32), (&str, u32, u32)
     let xn   = quotient.var("x");
     let nxn  = quotient.var("x\u{0304}");
     let prod = xn * nxn;