row_block class
The row_block class is an array of rows which has associated with
it a reference to an array of changes, and can recalculate itself from
those changes. For example, suppose that the variable c of type
vector<change> holds one lead of a method; then it is possible to
define a variable of type row_class which, once it is told what
the lead head is, will calculate the rows for one lead of the method.
Note that the row_block object does not store its own copy of the
vector<change> object from which it is calculated. This means
that the changes may be altered and the resulting rows recalculated
easily; it also means that the changes must not be deallocated while the
row_block object is in existence.