修正容易性 
参考定義
IEEE std 830-1998 (Modifiable)
An SRS is modifiable if, and only if, its structure and style are such that any changes to the requirements can
be made easily, completely, and consistently while retaining the structure and style. Modifiability generally
requires an SRS to
a) Have a coherent and easy-to-use organization with a table of contents, an index, and explicit cross-referencing;
b) Not be redundant (i.e., the same requirement should not appear in more than one place in the SRS);
c) Express each requirement separately, rather than intermixed with other requirements.
Redundancy itself is not an error, but it can easily lead to errors. Redundancy can occasionally help to make
an SRS more readable, but a problem can arise when the redundant document is updated. For instance, a
requirement may be altered in only one of the places where it appears. The SRS then becomes inconsistent.
Whenever redundancy is necessary, the SRS should include explicit cross-references to make it modifiable.
