References to defined names are enclosed in bracketing delimiters. 
The delimiter characters imply the type of the associated value as
shown in the table below.  A type error is raised if the type of a
reference does not match the type of the defined value.  The intent of
this mechanism is, again, to make sketch input files more
readable.
   
| Type | Reference | 
|---|---|
| scalar | id | 
| point | (id) | 
| vector | [id] | 
| transform | [[id]] | 
| drawable | {id} | 
| options | [id]or[id1,...,idN] | 
| tag | <id> | 
Note that square brackets [ ] are used both for vector and for options references. Details of
sketch syntax make it
impossible for these two reference types to be confused.  The
special multiple reference [id1,id2,...,idN]
acts as if the respective lists of options were concatenated.