Document TAXSIM implementation caveats

[tmm1]

TAXSIM replaces "cliffs" with dollar-for-dollar phase-outs. For example, the Earned Income Tax Credit is available only to filers with under $10,000 investment income. TAXSIM incorrectly phases out the EITC for filers whose investment income exceeds $10,000; for example, a single parent with $11k wages and $11k dividends is ineligible for the EITC, but taxsim shows $2,221 in 2021