lex ['<':'=':rest] = (Right LesserEqToken, rest)
lex ['>':'=':rest] = (Right GreaterEqToken, rest)
lex ['=':'=':rest] = (Right EqualsToken, rest)
lex ['<':'=':rest] = (Right LesserEqToken, rest)
lex ['>':'=':rest] = (Right GreaterEqToken, rest)
lex ['=':'=':rest] = (Right EqualsToken, rest)