Parse parses an observation from r into o using i.

Allowed is the 'allowed' test type.

None states that we haven't parsed a test type yet.

Required is the 'required' test type.

ErrBadState occurs if the parser somehow gets into an unknown state.

ErrBadStateCount occurs if the number of states is invalid.

ErrBadStateLine occurs if the state line is badly formed.

ErrBadSummary occurs if the summary line is badly formed.

ErrBadTestType occurs if the test-type line is badly formed.

ErrBadTransition occurs if the parser somehow performs a bad transition.

ErrInputEmpty occurs if the input to the parser runs out before anything is parsed.

ErrNoImpl occurs if the parser's impl is nil.

ErrNoStates occurs if the input to the parser runs out before any states are parsed.

ErrNoSummary occurs if the input to the parser runs out before the summary is parsed.

ErrNotEnoughStates occurs if the input to the parser runs out midway through the expected number of states.

ErrNoTest occurs if the input to the parser runs out before the start of a test is parsed.

StateLine is the struct that an implementation of the Herdtools parser must return when parsing a state line.

Impl describes the parser functionality that differs between Herdtools-style backends.

TestType is the type of test we're parsing.