3A. format
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An expression is a piece of code held together by operators, functions
and parentheses to form a value.
They mostly consist of:

- immediate values (see  2A ,...)
- operators (see  4A ,...)
- function calls (see  3D )
- parentheses () (see  3B )
- variables or variable-expressions (see  3C )

examples of expressions:
1
'hello'
$ABCD+(2*6)+Abs(a)
(a<1) OR (b>=100)