[Gmsh] Line of multiple segments

[Christophe Geuzaine]

Geordie McBain wrote:
> I just noticed that the Line command accepts more than two points.  In fact, it behaves like BSpline and Spline and accepts an expression-list.  This behaviour is quite useful in my application, but is undocumented (S. 5.1.2 `Lines' <http://geuz.org/gmsh/doc/texinfo/gmsh.html#Lines>).  Should and may I rely on this behaviour?
>    The behaviour is useful because I only need one line command for each chain of line segments, and because it applies a single identifier to the whole chain it is much easier to refer to the chain later, e.g. in the Line Loop and Physical Line commands.  It's also useful because I can simply swap Line, Spline, and BSpline in the input file without any other modification.  I attach three examples, differing only in this command.
>    If the behaviour is intended and may be relied on, perhaps the documentation for Line could be amended to read something like:

You can fairly safely use lines with multiple segments in 2D, but they 
are not supported in 3D (e.g. extruding a "multi-line" will lead to 
unexpected results). We chose not to document the feature for this reason...

> 
> %<---OLD
> Line ( expression ) = { expression, expression };
>     Creates a straight line segment. The expression inside the parentheses is the line segment's identification number; the two expressions inside the braces on the right hand side give identification numbers of the start and end points of the segment. 
> %<---PROPOSED
> Line ( expresison ) = { expression-list };
>     Creates a polygonal line.  The expression inside the parenthesis is the polygonal line's identification number; the expression-list on the right hand side should contain the identification numbers of all the polygonal line's vertices.
> %<---END
> 
> 
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-- 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine