I3st has the same representation as I1st

because it's only implemented (to the extent that it is implemented) for
fundamentals, so there's no concern about stuff like (1,1,1) as in I2st

spectro/src/lib.rs

@@ -876,7 +876,7 @@ impl Spectro {
         let mut ret = Vec::new();
         for nst in 0..nstop {
             match &states[nst] {
-                State::I1st(v) => {
+                State::I1st(v) | State::I3st(v) => {
                     // accept all zeros=ground state or a single 1=fund
                     if v.iter().sum::<usize>() > 1 {
                         continue;
@@ -887,7 +887,6 @@ impl Spectro {
                         continue;
                     }
                 }
-                State::I3st(_) => todo!(),
                 State::I12st { i1st: _, i2st: _ } => continue,
             }
             let vib1 = rotnst[(nst, ia)] - self.rotcon[ia];