Some selectivity types rely on a desired isomer

[qai222]

Currently EE ER DR are defined based on a desired isomer.

    enum SelectivityType {
      UNSPECIFIED = 0;
      CUSTOM = 1;
      EE = 2;  // Enantiomeric excess as a percentage (desired - undesired).
      ER = 3;  // Enantiomeric ratio. (x:1) (desired / undesired).
      DR = 4;  // Diasteromeric ratio. (x:1) (desired / undesired).
      EZ = 5;  // Alkene geometry ratio. (x:1) (E / Z)
      ZE = 6;  // Alkene geometry ratio. (x:1) (Z / E)
    }

It seems to me that this implies there are exactly two ProductCompound instances, and only one of them is desired. This could be ambiguous when there are other products for which the user has to look at the structures and decide which two of the products are enantiomers/diasteromers.