'Create a new expression.
:type name: string
:param name: The name of an expression for processing
during a search request.
:type value: string
:param value: The expression to evaluate for ranking
or thresholding while processing a search request. The
Expression syntax is based on JavaScript expressions
and supports:
* Single value, sort enabled numeric fields (int, double, date)
* Other expressions
* The _score variable, which references a document\'s relevance
score
* The _time variable, which references the current epoch time
* Integer, floating point, hex, and octal literals
* Arithmetic operators: + - * / %
* Bitwise operators: | & ^ ~ << >> >>>
* Boolean operators (including the ternary operator): && || ! ?:
* Comparison operators: < <= == >= >
* Mathematical functions: abs ceil exp floor ln log2 log10 logn
max min pow sqrt pow
* Trigonometric functions: acos acosh asin asinh atan atan2 atanh
cos cosh sin sinh tanh tan
* The haversin distance function
Expressions always return an integer value from 0 to the maximum
64-bit signed integer value (2^63 - 1). Intermediate results are
calculated as double-precision floating point values and the return
value is rounded to the nearest integer. If the expression is
invalid or evaluates to a negative value, it returns 0. If the
expression evaluates to a value greater than the maximum, it
returns the maximum value.
The source data for an Expression can be the name of an
IndexField of type int or double, another Expression or the
reserved name _score. The _score source is
defined to return as a double from 0 to 10.0 (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.
:return: ExpressionStatus object
:rtype: :class:`boto.cloudsearch2.option.ExpressionStatus` object
:raises: BaseException, InternalException, LimitExceededException,
InvalidTypeException, ResourceNotFoundException'

def create_expression(self, name, value):

    data = self.layer1.define_expression(self.name, name, value)
    data = data['DefineExpressionResponse']['DefineExpressionResult']['Expression']
    return ExpressionStatus(self, data, self.layer1.describe_expressions)
