Parse expression with binary and unary operators, reserved words, and without parentheses

I'm trying to parse expressions made of the binary operator +, the unary operator not and identifiers that can be any alphabetical string that isn't not

from pyparsing import (
    CaselessKeyword,
    Combine,
    Word,
    alphas,
    opAssoc,
    infixNotation,
)

identifier = Combine(~CaselessKeyword('not') + Word(alphas))
expression = infixNotation(identifier, [
  ('+', 2, opAssoc.LEFT),
  (CaselessKeyword('not'), 1, opAssoc.RIGHT),
]

Running

expression.parseString('a + (not b)')

gives what I expect

[['a', '+', ['not', 'b']]]

However, without the parentheses

expression.parseString('a + not b')

I only get the first token:

['a']

How can I define the language to work as I would like without the parentheses?

(In the real case there are more operators and reserved words: this is a step towards parsing the S3 Select language)

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