C STACK     STACK FOR EXPRESSION ENCODING AND DECODING
C
C LEV       CURRENT LEVEL IN TREE
C IOP       STACK OF OPERATIONS TO CURRENT TREE POSITION
C IVAL      ADDRESS FOR RESULT OF OPERATION
C IREF      INDICATES WHETHER RESULT OF OPERATION IS STORED IN REGISTERS 
C           OR IN DATA ARRAY
C
      INTEGER         LEV, IOP(50), IVAL(50), IREF(50)
C
      COMMON /STACK/  LEV, IOP, IVAL, IREF