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In most cases, FlexPDE notation is simple text as in a programming language.
•  Differentiation, such as du/dx, is denoted by the form dx(u). All active coordinate names are recognized, as are second derivatives like dxx(u) and differential operators Div, Grad and Curl. 
•  Names are NOT case sensitive. "F" is the same as "f". 
•  Comments can be placed liberally in the text. Use { } to enclose comments or ! to ignore the remainder of the line. 
Note: See the Problem Descriptor Reference chapter on Elements for a full description of FlexPDE notation.