= Explicit block bound for alphabet insensitivity
{title2=$L_i=r^{k^{P_i+d-i}}$}
For r colours, k letters and d variable blocks, set $P_i=\sum_{j<i}L_j$ and $L_i=r^{k^{P_i+d-i}}$. Process blocks from right to left. In block i, the $L_i+1$ words $a^t b^{L_i-t}$ have <colour profiles> indexed by all $k^{P_i}$ earlier words and $k^{d-i}$ later variable assignments. Two profiles agree, giving a nonempty variable interval insensitive to exchanging a and b. Previously obtained insensitivity holds for all earlier words and survives their restriction. This constructs the <combinatorial subspace> required by the <alphabet insensitivity lemma>.
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