Class: question_attempt_step  - X-Ref

Stores one step in a {@see question_attempt}.

The most important attributes of a step are the state, which is one of the
{@see question_state} constants, the fraction, which may be null, or a
number bewteen the attempt's minfraction and maxfraction, and the array of submitted
data, about which more later.

A step also tracks the time it was created, and the user responsible for
creating it.

The submitted data is basically just an array of name => value pairs, with
certain conventions about the to divide the variables into five = 2 x 2 + 1 categories.

Variables may either belong to the behaviour, in which case the
name starts with a '-', or they may belong to the question type in which case
they name does not start with a '-'.

Second, variables may either be ones that came form the original request, in
which case the name does not start with an _, or they are cached values that
were created during processing, in which case the name does start with an _.

In addition, we can store 'metadata', typically only in the first step of a
question attempt. These are stored with the initial characters ':_'.

That is, each name will start with one of '', '_', '-', '-_' or ':_'. The remainder
of the name was supposed to match the regex [a-z][a-z0-9]* - but this has never
been enforced. Question types exist which break this rule. E.g. qtype_combined.
Perhpas now, an accurate regex would be [a-z][a-z0-9_:]*.

These variables can be accessed with {@see get_behaviour_var()} and {@see get_qt_var()},
- to be clear, ->get_behaviour_var('x') gets the variable with name '-x' -
and values whose names start with '_' can be set using {@see set_behaviour_var()}
and {@see set_qt_var()}. There are some other methods like {@see has_behaviour_var()}
to check wether a varaible with a particular name is set, and {@see get_behaviour_data()}
to get all the behaviour data as an associative array. There are also
{@see get_metadata_var()}, {@see set_metadata_var()} and {@see has_metadata_var()},

Class: question_attempt_pending_step  - X-Ref

A subclass of {@see question_attempt_step} used when processing a new submission.

When we are processing some new submitted data, which may or may not lead to
a new step being added to the {@see question_usage_by_activity} we create an
instance of this class. which is then passed to the question behaviour and question
type for processing. At the end of processing we then may, or may not, keep it.

Class: question_attempt_step_read_only  - X-Ref

A subclass of {@see question_attempt_step} that cannot be modified.

Class: question_null_step  - X-Ref

A null {@see question_attempt_step} returned from
{@see question_attempt::get_last_step()} etc. when a an attempt has just been
created and there is no actual step.

Class: question_attempt_step_subquestion_adapter  - X-Ref

This is an adapter class that wraps a {@see question_attempt_step} and
modifies the get/set_*_data methods so that they operate only on the parts
that belong to a particular subquestion, as indicated by an extra prefix.