The method SH_NodeFragment_get_attr provides a pointer to an Attr, by
its index. Note, that it directly points to the internal data, instead
of copying the data to a new Attr, which would be unneccessary overhead,
if only reading access is needed. That's why it is also a const pointer.
If the user intends to modify it, a copy should be taken via
SH_Attr_copy.

Multiple insert methods allow either to add an existing Attr, or to
create a new one implicitly. If the Attr is not already used beforehand,
it is more efficient to call the attr_new methods. Also an old Attr is
freed, after it was inserted, thus it can't be used afterwards. This is
neccessary, as for efficiency reasons an array of Attr is used directly,
instead of the indirect approach of storing a pointer of Attr. This
means, that the contents of the Attr has to be copied to the internal
structure. If the old Attr would be left unfreed, there would be two
Attrs, the original one and the implicit one, referring to the same
data, which would lead to at least data corruption, or undefined
behaviour like a double free, which would be a serious threat for a
library which is to be used on a webserver. ...
For each of the two insert modes, there is a method to prepend, append
or insert at a specific position. An incorrect position is handled
inside of the external method and an E_VALUE is thrown. The internal
method doesn't handle this, so special care must be taken to not make
undefined behaviour. However enforcing this check would be unneccessary
overhead for the prepend and append methods, which are known to have
correct indicies, as well for other internal methods, where the internal
method may be used.

The tests for the new methods are still missing, as well as the methods
to remove an Attr. As the implementation is already a bit dated, the
changes are commited now anyways.