Constants

ISO 10303-11 (clause 7.2, table 3) defines built-in constants; ExpressLexer emits Keyword.Constant for all of them. The indeterminate value ? is included here rather than treated as an operator, and is matched before the operator characters so it never becomes punctuation or an operator.

built-in constants

?
SELF
CONST_E
PI
FALSE
TRUE
UNKNOWN

In context:

constants in context

SCHEMA constant_probe;
  CONSTANT
    e     : REAL := CONST_E;        -- CONST_E
    pi_n  : REAL := PI;             -- PI
  END_CONSTANT;

  ENTITY point;
    ok    : BOOLEAN := TRUE;        -- TRUE
    bad   : BOOLEAN := FALSE;       -- FALSE
  WHERE
    wr1   : point <> UNKNOWN;       -- UNKNOWN
    wr2   : point = SELF;           -- SELF
  END_ENTITY;
END_SCHEMA;

? is the indeterminate constant used for an unknown value or an open bound:

the indeterminate constant

x := ?;                 -- a value that is not known
SET [0:?] OF item;      -- an open upper bound

Note CONST_E here is the mathematical constant named in table 3, and must not be read as the declaration head CONSTANT from the Declarations page.