A question on the integral of a step function

In Apostol's "Mathematical Analysis", a step function on closed interval [a,b] is defined as follows (Page 148):

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A step function on a general interval I and its integral over I are defined as follows (Page 253):

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But in the proof of Theorem 10.10 in page 259 (see figure below), the underlined sentence asserts the existence of $\displaystyle \int _J s_n^+$ over an arbitrary subinterval J. If $\displaystyle s_n^+$ assumes a constant value $\displaystyle \ne 0$ in some $\displaystyle [a,b]\subseteq I$, while J=(c,d) with c<a and a<d<b, then $\displaystyle s_n^+$ is not a step function on J because we can not find a closed interval in J such that $\displaystyle s_n^+$ is 0 outside of this closed interval. In turn, the integral $\displaystyle \int_J s_n^+$ is undefinable. How to handle this problem? Thanks!

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