Nothing special about levels attribute - no point in making it part of the interface.

...y/src/com/oracle/truffle/r/library/utils/TypeConvert.java

...src/com/oracle/truffle/r/nodes/builtin/base/AsVector.java

...in/src/com/oracle/truffle/r/nodes/builtin/base/Match.java

...racle/truffle/r/nodes/builtin/base/PrettyPrinterNode.java

...n/src/com/oracle/truffle/r/nodes/builtin/base/Repeat.java

...in/src/com/oracle/truffle/r/nodes/builtin/base/Split.java

...c/com/oracle/truffle/r/nodes/builtin/base/UpdateAttr.java

...oracle/truffle/r/nodes/builtin/base/UpdateAttributes.java

...com/oracle/truffle/r/nodes/builtin/base/UpdateLevels.java

.../com/oracle/truffle/r/nodes/binary/BinaryBooleanNode.java

...untime/src/com/oracle/truffle/r/runtime/data/RFactor.java

...untime/src/com/oracle/truffle/r/runtime/data/RVector.java

...com/oracle/truffle/r/runtime/data/closures/RClosures.java