## Description

An independentVarRef more fully describes the input mapping of
the function by pointing to a separate breakpoint definition
element.
An optional extrapolate attribute describes how to extrapolate
the output value when the input value exceeds specified values
(default is 'neither,' meaning the value of the table is held
constant at the nearest defined value). An optional interpolate
attribute indicates how to perform the interpolation within the
table (supporting discrete, linear, cubic or quadratic
splines). There are three different discrete options: 'discrete'
means nearest-neighbor, with an exact mid-point value being
rounded in the positive direction; 'floor' means the function
takes on the value associated with the next (numerically) higher
independent breakpoint as soon as original value is exceeded;
and 'ceiling' means the function holds the value associated with
each breakpoint until the next (numerically) higher breakpoint
value is reached by the independent argument. The default
interpolation attribute value is 'linear.'
Note also that the option min and max attributes serve to limit
the numeric range of the independent (function input) variable;
these attributes are independent of and serve as additional
range contraints with respect to any minValue or maxValue
attribute associated with the independent variable itself.
This element allows reuse of common breakpoint values for many
tables but with possible differences in interpolation or
extrapolation for each use.