If and are class 1, then is also class 1. Just choose a -edge coloring of and a -edge coloring of , and then assign each edge of the color of the edge it projects to in the appropriate projection map. This gives a proper edge coloring of , and .
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If
and 
are class 1, then 
is also class 1. Just choose a 
-edge coloring of 
and a 
-edge coloring of 
, and then assign each edge of 
the color of the edge it projects to in the appropriate projection map. This gives a proper 
edge coloring of 
, and 
.