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In [1], the statements of Non-Proposition 2.4 (on page 438) and of Propositions 2.5 and 2.6 (on page 439) have been published in incomplete form. The complete statements read as follows:

Non-Proposition 2.4 We have inline image if and only if β satisfies one of the following conditions:

  1. β is a theorem.
  2. inline image.
  3. There is a finite, non-empty sequence inline image of formulas such that inline image and each inline image satisfies one of the conditions (a), (b) or (c).

Proposition 2.5 We have inline image if and only if β satisfies one of the following conditions:

  1. β is a theorem.
  2. inline image.
  3. inline image for some finite, non-empty sequence inline image of theorems.

Proposition 2.6 We have inline image if and only if α and β are both theorems, or inline image.

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