start expression evaluation
re-design
re-design
tidy up contracts
implementing parser
add test cases to parser
start 'add' operand
making parser
making mocks for parser
tidy up
finish parser
re-design expression builder
refactor design
abstract design and writing acceptance test
optimize abstract concepts
build acceptance test
tidy up and prepare for expression
writing unit test for expression
pass compilation for expression test
redesign model and update mock