This attribute indicates
whether a 
Mapping is an instance of a class that always represents a linear
transformation. Note, some Mapping classes can represent linear or non-linear
transformations (the 
MathMap class for instance). Such classes have a zero value for
the IsLinear attribute. Specific instances of such classes can be tested for linearity
using the 
astLinearApprox function. AST_LINEARAPPROX routine.  
  Mapping  
 All Mappings have this attribute.  
  The IsLinear value for a CmpMap is determined by the classes of the
encapsulated Mappings. For instance, a CmpMap that combines a 
ZoomMap and a 
ShiftMap
will have a non-zero value for its IsLinear attribute, but a CmpMap that contains a
MathMap will have a value of zero for its IsLinear attribute.  
 The IsLinear
value for a Frame is 1 (since a Frame is equivalent to a 
UnitMap).  
 The
IsLinear value for a FrameSet is obtained from the Mapping from the base Frame to the
current Frame.