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CHAPTER I [:] Notions from Set Theory [...] 2. OPERATIONS ON SETS. "end quote
These pages assume that the reader has the book and is reading along.    
topic index     contents
Please read these notes from the beginning to here, and then paragraphs 4 and 5 of 2.

EXAMPLES FROM THE NUMBER LINE: UNION & COMPLEMENT

The symbols     and     will be formally defined in chapter 2. However, we can use them here because we can understand their meaning from the understanding of numbers that we already have. The underline should be read as 'or equal to' and the angle bracket part points to the smaller number. The big end is at the big number, and we can read both symbols either from the pointy end to the big end as 'is less than or equal to' or from the big end to the pointy end as 'is greater than or equal to'. I recommend reading from the pointy end to the big end even when that means reading from right to left.

Consequently:
Set

is
the set of   x   in   S   such that   1   is less than or equal to   x.

and:
Set

is
the set of   x   in   S   such that   0   is less than or equal to   x   and   x   is less than or equal to   1.

Similarly,
in the union of   X   and   Y   we have the condition that   0   is less than or equal to   x.
And, in the complement of   X,  we have the condition that   x   is less than   1.

on to reading 3