The layer_expr variable contains a parsed layer expression. A layer expression is an expression consisting of layer names and logical operators. A layer expression is evaluated within a certain region, representing part of a physical layout, and returns the regions where the layer expression is ``true''.
A layer_expr is a piece of compiled code that can execute very quickly. Functions that accept a layer_expr argument will generally also accept a string containing the layer expression, and will compile the string before use. If an expression is to be used multiple times, if is far more efficient to pass a layer_expr variable.
These variables can not be assigned, and no operators can be applied. They can be passed to functions only.
If passed to the Print family of functions, the layer expression string will be printed.