Each program is an expression to be evaluated,
or conversely, every expression can serve as a Pico program. Expressions are divided
into three sorts: definition expressions (with a colon), assignment expressions
(with a colon-equals) and 'reference expressions' where one refers a variable,
a function or a table. Each of these three 'actions' are possible with the three
kinds of Pico values: tables, values and functions. Making the full 'cross product'
of these two concepts (the values that walk around and the operations) gives nine
"fundamental actions" in Pico, which are represented by the concrete grammar.
This corresponds to the following table of the Pico syllabus: