Constant TARGET_SUM 

pub const TARGET_SUM: usize = _; // 195usize

A WOTS encoding (e_0, .., e_{V-1}) is valid exactly when every e_i < CHAIN_LENGTH, the digits sum to this, and the two leftover digest bits are zero.

The signer grinds the randomness until the encoding is valid, which is what replaces the checksum chains. 195 sits above the mean of 147 so the verifier walks fewer chain steps.