Unknown Attribute Values

What if some examples are missing values of some $A$?
Use training example anyway, sort through tree:
If node $n$ tests $A$, assign most common value of $A$ among other examples sorted to node $n$, or
assign most common value of $A$ among other examples with same target value, or
assign fraction $p_{i}$ of example to each descendant in tree.
Classify new examples in same fashion