$$(1-a)-[1-(a+3)]$$
$3$
$$1-a-\left(1-a-3\right)$$
$$1-a-\left(-2-a\right)$$
$$1-a-\left(-2\right)-\left(-a\right)$$
$$1-a+2-\left(-a\right)$$
$$1-a+2+a$$
$$3-a+a$$
$$3$$
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