Function bd0_terms 

pub const fn bd0_terms(precision: PrecisionPolicy) -> usize

Terms of the odd series in [bd0] by tier, inside |v| < 1/5.

The dropped term is 2k v^{2T+1}/(2T+1), an absolute error in the exponent and so a relative error of k times 0.2^{2T+1}/(2T+1) in the density: 12 terms is k * 7e-19, 8 is k * 1e-13, 5 is k * 4e-9. For comparison the direct form at the window edge is off by about k * eps / 2 from its own cancellation, so 12 terms matches it in binary64 and 5 in binary32.