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9.6 Parameters are constant
Within function definition bodies, the parameters may be used like any
other variable. But the parameters are constant in the sense that
they can’t be assigned to (i.e., can’t appear on the left side of an
assignment (=) statement). In other words, their value remains
constant throughout the function body. Attempting to assign a value
to a function parameter value will raise a compile-time error.14
Local variables may be declared at the top of the function block and scope as usual.
Despite being declared constant and appearing to have a pass-by-value syntax in Stan, the implementation of the language passes function arguments by constant reference in C++.↩