RESO RCP-019

ANTLR 4 implementation of the RESO RCP-019 grammar, with backwards-compatibility support for RETS 1.9 Validation Expressions.

20180506 - using the included `rcp019.g4` file, the following expressions should parse:

Basic arithmetic
3 + 5
Bracketed RETSNAMEs
[ListPrice] > 0
RCP61 added support for unbracketed RETSNAMEs
ListPrice > 0
To get current value of any field, just use the DICTNAME of that field
ListPrice
RETS had support for the LAST keyword, which now works on all fields
to get the old value
LAST Status != "Sold"
Which is useful for field change detection
ListPrice != LAST ListPrice
Function calls are also supported, as long as they don't use
any of the RETSNAME keywords
foo()
foo ('bar', 'baz', 42)
Lists of expressions are also supported
(ListPrice, Status, 3+5, LAST Status, (1, 2, 3))
Supports complex expressions with grouping
ListPrice > 5.01 .AND. (1, 2, 3) .CONTAINS. 3 .OR. (Status .IN. ('Active', 'Pending') .AND. .USERLEVEL. != "Admin")
Which parses differently than
ListPrice > 5.01 .AND. (1, 2, 3) .CONTAINS. 3 .OR. Status .IN. ('Active', 'Pending') .AND. .USERLEVEL. != "Admin"

RCP-019.1 added support for collections which include LIST() and SET(). These functions take 0 or more items. Backwards compatibility with the previous list syntax of () has been preserved.

Expression	Result	Comments
`LIST(1, 2, 2, 3)`	`LIST(1, 2, 2, 3`	Duplicate items are preserved.
`SET(1, 2, 2, 3)`	`SET(1, 2, 3)`	Duplicate items are removed.

RCP-019.1 also added support for DIFFERENCE(), INTERSECTION(), and UNION(), which are intended to produce types in the following manner:
- Operations on homogeneous collections of LIST() or SET() should return LIST() or SET(), respectively. For example, UNION(LIST('a', 'b', 3+5), LIST(6)) should return LIST('a', 'b', 6, 8).
- Operations on heterogenous collections of LIST() or SET(), for instance DIFFERENCE(LIST(1, 2, 3), SET(3)) should return LIST().

These special functions require at least two arguments of type LIST() or SET()

Expression	Result	Comments
`DIFFERENCE(LIST(1, 2, 3), LIST(1, 2))`	`LIST(3)`	Collection operators require two or more `LIST()` or `SET()` arguments.
`UNION(LIST(1, 2), SET(3))`	`LIST(1, 2, 3)`	Arguments of type `LIST()` are converted to `SET()`.
`INTERSECTION(SET(DIFFERENCE(LIST(1, 2, 3), SET(1))), SET(2))`	`SET(2)`	Since the return type of `collection` operators is `SET()` or `LIST()`, they can be composed.