UDAs

- UDA definitions that have the same name as a core attribute are not
  permitted.

src/columns/Column.cpp

@@ -173,6 +173,9 @@ void Column::uda (std::map <std::string, Column*>& all)
   std::map <std::string, int>::iterator uda;
   for (uda = udas.begin (); uda != udas.end (); ++uda)
   {
+    if (all.find (uda->first) != all.end ())
+      throw format (STRING_UDA_COLLISION, uda->first);
+
     Column* c = Column::uda (uda->first);
     all[c->_name] = c;
   }

src/en-US.h

@@ -687,6 +687,7 @@
 #define STRING_INFINITE_LOOP         "Terminated substitution because more than {1} changes were made - infinite loop protection."
 #define STRING_UDA_TYPE              "User defined attributes may only be of type 'string', 'date', 'duration' or 'numeric'."
 #define STRING_UDA_NUMERIC           "The value '{1}' is not a valid numeric value."
+#define STRING_UDA_COLLISION         "The UDA named '{1}' is the same as a core attribute, and is not permitted."
 #define STRING_INVALID_MOD           "The '{1}' attribute does not allow a value of '{2}'."
 
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