What is UFL expression

[Alpha-Legion]

I encountered this in the documentation about immersed mesh generation. But I can not find its definition and usage.

[dham]

A UFL expression is a primal mathematical expression written in UFL. So a Function or a TestFunction or a TrialFunction are UFL expressions, as are any algebraic operations on them. E.g. if f is a Function then f**2 and sin(f) are also UFL expressions. The key things that are not UFL expressions include integral Forms - so the result of multiplying a UFL expression by dx is a Form which is not an expression.

[Alpha-Legion]

Thanks!