This is an experimental alternate Parser implementation, with a view to facilitating the integration
of Part21 and Part28 exchange structures with other schema (such as AP203, AP214 and AP242).

The idea is to build a base Part21 parser implementation implemented using Ply (such as the one you
can now find in SCL.Part21 in this branch).

Exp2Python would be enhanced to subclass this parser and implement parser rules based on Part21
type / parameter mapping of schemas.

Exp2Python output Entity classes would be minimal, implementing all calculated attributes and
automatic type conversion from Part21 parameter types to the schema defined types.  DERIVED
attributes would be implemented in the output class (as they are now).

Validating and non-validating versions of the parser would be generated.

For the validating parser all type checking / compatibility would be implemented by parser rules and
callbacks.  Callbacks would be run after the first parser run, or sooner if a way could be found to
trigger them when appropriate.

Some pseudo code is given below (see comments):

import SCL.Part21

# entity classes would be fully implemented by exp2python
class RepresentationItem:
    def __init__(self, name):
        pass

# complete implementation not shown here!
class GeometricRepresentationItem(RepresentationItem):
    def __init__(self, name):
        RepresentationItem.__init__(self, name)

class Curve(GeometricRepresentationItem):
    def __init__(self, name):
        GeometricRepresentationItem.__init__(self, name)

class Conic(Curve):
    def __init__(self, name, position):
        Curve.__init__(self, name, position)

class Circle(Conic):
    def __init__(self, name, position, radius):
        Conic.__init__(self, name, position, radius)
        

class AP203Parser(SCL.Part21.Parser):
    entities = ['ABSORBED_DOSE_MEASURE_WITH_UNIT',
                'ABSORBED_DOSE_UNIT',
                ...,
                'AXIS1_PLACEMENT',
                'AXIS2_PLACEMENT_2D',
                'AXIS2_PLACEMENT_3D',
                ...,
                'CIRCLE',
                'CIRCULAR_RUNOUT_TOLERANCE',
                'YEAR_MONTH',
                ...,
                'ZONE_STRUCTURAL_MAKEUP']

    def __init__(self, ...):
        SCL.Part21.Parser.__init__(self)
        # schema entities would be passed to the underlying lexer to facilitate exact rule matches
        self.lexer.register_entities(self.entities)    
    
    # validating form
    def p_circle_entity(self, p):
        r"""simple_record : CIRCLE '(' STRING ',' ENTITY_INSTANCE_NAME ',' REAL ')'"""
        p[0] = Circle(p[3], p[5], p[7])
        self.add_instance_validation(p[5], lambda x: isinstance(x, Axis2Placement))

    # non validating form
    def p_circle_entity(self, p):
        r"""simple_record : CIRCLE '(' parameter_list ')'"""
        p[0] = Circle(*p[3])