public Builder addReference(SymbolReference reference) {
            references.get().add(Objects.requireNonNull(reference));
            return this;
        }
Add a symbol reference to indicate that this symbol points to
or contains references to other symbols.

@param reference Symbol reference to add.
@return Returns the builder.
smithy-codegen-core/src/main/java/software/amazon/smithy/codegen/core/Symbol.java