Polybuilder allows me to build sequences that are impossible.

**Tool:**
test.glycam.org/pb

**Bug Description:**
I was able to build this:
DGalpa1-9DNeup5Ac**a2-2DKDNpa2**-[2DGalpa1-2DGalpb1-]<2>OH

**To Reproduce:**
Point and click interface in polybuilder.

**Expected behavior**
I should not be able to create a2-2D linkages in the "head", note that for KDN it forms a 2-* linkage, so there are now two residues connecting to the KDN 2 position.

**Screenshots**

**Importance**
Not high priority, gmml will give a somewhat useful error, and most people building these oddball carbs should know this.