One example is in the CSV parser 

 `+		for i := range attrs {
 +			instances.SetAttrStr(rowCounter, i, record[i])
 +		}
`
This calls something in *Instances*
+// SetAttrStr sets the system-representation value of row in column attr
 +// to value val, implicitly converting the string to system-representation
 +// via the appropriate Attribute function.
 +func (inst *Instances) SetAttrStr(row int, attr int, val string) {
 +	// Set an attribute on a particular row from a string value
 +	a := inst._Attributes[attr]
 +	sysVal := a.GetSysValFromString(val)
 +	inst._Storage.Set(row, attr, sysVal)
 +}

And *voila*, the CSV reader can add new values to a given *Attribute* painlessly. Once the CSV's been read, it might be useful to provide a guarantee that it can't change - especially if you have lots of goroutines accessing a given *Attribute*.