'Create a new rank expression.
:type rank_name: string
:param rank_name: The name of an expression computed for ranking
while processing a search request.
:type rank_expression: string
:param rank_expression: The expression to evaluate for ranking
or thresholding while processing a search request. The
RankExpression syntax is based on JavaScript expressions
and supports:
* Integer, floating point, hex and octal literals
* Shortcut evaluation of logical operators such that an
expression a || b evaluates to the value a if a is
true without evaluting b at all
* JavaScript order of precedence for operators
* Arithmetic operators: + - * / %
* Boolean operators (including the ternary operator)
* Bitwise operators
* Comparison operators
* Common mathematic functions: abs ceil erf exp floor
lgamma ln log2 log10 max min sqrt pow
* Trigonometric library functions: acosh acos asinh asin
atanh atan cosh cos sinh sin tanh tan
* Random generation of a number between 0 and 1: rand
* Current time in epoch: time
* The min max functions that operate on a variable argument list
Intermediate results are calculated as double precision
floating point values. The final return value of a
RankExpression is automatically converted from floating
point to a 32-bit unsigned integer by rounding to the
nearest integer, with a natural floor of 0 and a ceiling
of max(uint32_t), 4294967295. Mathematical errors such as
dividing by 0 will fail during evaluation and return a
value of 0.
The source data for a RankExpression can be the name of an
IndexField of type uint, another RankExpression or the
reserved name text_relevance. The text_relevance source is
defined to return an integer from 0 to 1000 (inclusive) to
indicate how relevant a document is to the search request,
taking into account repetition of search terms in the
document and proximity of search terms to each other in
each matching IndexField in the document.
For more information about using rank expressions to
customize ranking, see the Amazon CloudSearch Developer
Guide.
:raises: BaseException, InternalException, LimitExceededException,
InvalidTypeException, ResourceNotFoundException'

def create_rank_expression(self, name, expression):

    data = self.layer1.define_rank_expression(self.name, name, expression)
    return RankExpressionStatus(self, data, self.layer1.describe_rank_expressions)
