anchor.diff:  
     implicit none
   
  -  ! Versioning numbers
  +  ! ============================================================================
  +  ! VERSIONING NUMBERS
  +
     integer, parameter :: VERSION_MAJOR   = 0
     integer, parameter :: VERSION_MINOR   = 3
     integer, parameter :: VERSION_RELEASE = 3
   
  -  ! Physical constants
  +  ! ============================================================================
  +  ! ADJUSTABLE PARAMETERS 
  +
  +  ! NOTE: This is the only section of the constants module that should ever be
  +  ! adjusted. Modifying constants in other sections may cause the code to fail.
  +
  +  ! Monoatomic ideal-gas scattering treatment threshold
  +  real(8), parameter :: FREE_GAS_THRESHOLD = 400.0
  +
  +  ! Used for surface current tallies
  +  real(8), parameter :: TINY_BIT = 1e-8
  +
  +  ! Maximum number of collisions/crossings
  +  integer, parameter :: MAX_EVENTS = 10000
  +  integer, parameter :: MAX_SAMPLE = 100000
  +
  +  ! Maximum number of words in a single line, length of line, and length of
  +  ! single word
  +  integer, parameter :: MAX_WORDS    = 500
  +  integer, parameter :: MAX_LINE_LEN = 250
  +  integer, parameter :: MAX_WORD_LEN = 150
  +  integer, parameter :: MAX_FILE_LEN = 255
  +
  +  ! ============================================================================
  +  ! PHYSICAL CONSTANTS
  +
     real(8), parameter ::            &
          PI           = 3.1415926535898_8, & ! pi
          MASS_NEUTRON = 1.0086649156,      & ! mass of a neutron
  
positive.code:        & SENSE_POSITIVE = 1, &
         & SENSE_NEGATIVE = -1
  
    ! Used for surface current tallies
    real(8), parameter :: TINY_BIT = 1e-8
  
    ! Maximum number of collisions/crossings
    integer, parameter :: MAX_EVENTS = 10000
    integer, parameter :: MAX_SAMPLE = 100000
  
    ! Codes for read errors -- better hope these numbers are never used in an
    ! input file!
    integer, parameter :: ERROR_INT  = -huge(0)
    real(8), parameter :: ERROR_REAL = -huge(0.0_8) * 0.917826354_8
  
    ! Source types
    integer, parameter ::   &
         SRC_BOX     = 1, & ! Source in a rectangular prism
         SRC_CELL    = 2, & ! Source in a cell
         SRC_SURFACE = 3    ! Source on a surface
  
    integer, parameter ::        &
         PROB_SOURCE      = 1, & ! External source problem
         PROB_CRITICALITY = 2    ! Criticality problem
  
    ! Interpolation flag
    integer, parameter ::     &
  
negative.code:        IN_TOP    = 5,   &
         OUT_TOP   = 6
  
    ! ============================================================================
    ! MISCELLANEOUS CONSTANTS
  
    ! Codes for read errors -- better hope these numbers are never used in an
    ! input file!
    integer, parameter :: ERROR_INT  = -huge(0)
    real(8), parameter :: ERROR_REAL = -huge(0.0_8) * 0.917826354_8
  
    ! Source types
    integer, parameter ::   &
         SRC_BOX     = 1, & ! Source in a rectangular prism
         SRC_CELL    = 2, & ! Source in a cell
         SRC_SURFACE = 3    ! Source on a surface
  
    integer, parameter ::        &
         PROB_SOURCE      = 1, & ! External source problem
         PROB_CRITICALITY = 2    ! Criticality problem
  
    ! Unit numbers
    integer, parameter :: UNIT_LOG   = 11 ! unit # for writing log file
  