The definition is,
I rewrite it as $$(L[y_1] = L[y_2] = 0) \rightarrow (L[c_1y_1 + c_2y_2] = 0)$$.
However, I also wonder, whether it could also be rewritten as,
$$(L[c_1y_1 + c_2y_2] = 0) \rightarrow (L[y_1] = L[y_2] = 0) $$
And thus, combining, the two cases,
Principle of superposition...