Its purpose is to define a boundary condition for a Partial Differential Equation (PDE).
The general syntax is
onbdy(ib1, ib2,...) id(u)+<expression>*dnu(u) = g onbdy(ib1, ib2,...) u = gwhere
ib's are boundary identification numbers, <expression> is a generic
expression and g a generic function.
The term id(u) may be absent as in -dnu(u)=g .
dnu(u) represents the conormal of the PDE, i.e.

when the PDE operator is
