Examining the equation for the GP between two points on slide 84 (time code 33:32) of @enkimute’s presentation:

How is x_1\mathbf{e}_{01}+y_1\mathbf{e}_{20}-x_2\mathbf{e}_{01}-y_2\mathbf{e}_{20}-1 = 1+(x_2-x_1)\mathbf{e}_{01} + (y_2-y_1)\mathbf{e}_{20} a valid manipulation? For one, I can’t for the life of me see how the -1 in the scalar part on the LHS could become a 1 in the RHS.