Drift name mangling with superposition error.

[DavidSagan]

Example: bend b1 superimposed upon drift d:

      3  "d!1!0"        Drift               0.100000   0.600000 ->    0.700000
      4  "b1"           Bend                0.200000   0.700000 ->    0.900000
      5  "d!1"          Drift               0.300000   0.900000 ->    1.200000

[DavidSagan]

Fixed.