Definition 1.13. Let $L_1,\dots ,L_k$ be combinatorial lines. We call them focussed if $L_i^{+}=f$ for all $i\in [k]$. For a fixed colouring, they are colour focused if they are focused and $L_i\setminus \{L_i^{+}\}$ monochromatic for each $i$, and they have different colours.