extending dictonary

[nospam]

Is there any way to extend the dictonary in such manner that I can 
insert muliplay value to each keys and return one of the value as the 
default value. I would like to have similar syste that I drawed out below.

tree[nucelotide_postionc][nucleotide]=default(value subtree) This should 
be returned when doing lookup without any
tree[nucelotide_postionc][nucleotide]=Postive value
tree[nucelotide_postionc][nucleotide]=Negative value
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