Invalid values for various types.
Many Elgrint types have special invalid values, which create a subset of the corresponding type's legal values. Invalid values are treated differently from valid values of that type by many functions (but not all). Most often such functions ignore invalid values or report an error. The exact behavior is specified for each individual function in this Reference.
The validity of a value is defined by the global IsValid function, which returns true for valid values and false for invalid ones. See IsValid for the exact definition of validity for each supported type.
Among all the invalid values for a given type, there are three with even more special meanings. These values have their own names: NoneXXXX, AutoXXXX, and SameXXXX. For example, MPoint has the corresponding NonePoint, AutoPoint, and SamePoint values. The meaning of these special invalid values differs from function to function. Some functions treat these special values no differently than other invalid values. But as a general rule of thumb, if a function parameter equals AutoXXXX, the function tries to change this parameter to some predefined or preferred value; if a parameter equals SameXXXX, the function tries to preserve the existing value of the suitable internal parameter. The NoneXXXX is often used as a return value to indicate that the function has failed.
See IsValid for more information.
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