Lemma 3.3.2. [04PU]

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Lemma 3.3.2.

Let $W_{q}$ be a wing over the edge $e_{lh}$ , for $l\in\{i,j,k\}$ and assume $i,j,k,h$ all distinct. Then the retraction $\rho_{\mathscr{X}_{ijk}}:W_{q}\rightarrow e_{lh}$ is the contraction of $W_{q}$ to the edge $e_{lh}$ parallel to the edge $e_{lq}$ :

	$\displaystyle\rho_{\mathscr{X}_{ijk}}:W_{q}$	$\displaystyle\rightarrow e_{lh}$
	$\displaystyle(x,y)$	$\displaystyle\mapsto(x-y,0)$