def add(schema_table)
      if schema_table.kind_of? Schema::Fact
        collection = @facts
      elsif schema_table.kind_of? Schema::Dimension
        collection = @dimensions
      end
      
      add_to_collection collection, schema_table
    end
def add(schema_table)
      if schema_table.kind_of? Schema::Fact
        collection = @facts
      elsif schema_table.kind_of? Schema::Dimension
        collection = @dimensions
      end
      
      add_to_collection collection, schema_table
    end
Adds a prebuilt schema table to the schema

 Schema tables may not be dupliates of already present tables in
 the schema.

 TODO: figure out how to deal with linked dimensions when adding
 facts.