in Freudenthal's words:
'the justification of the numerical operations and their laws by the simplicity of the algebraic description of geometrical figures and relations'
a rule e.g. y = 3 - x generates some points, by inputting some positive x values
- initially less than or equal to 3
- then bigger than 3
- then negative ("where does the line say the result should be?")
some (directed number) calculation results emerge
this work is better managed with talk but the resources intend to nudge in the desired direction