Construct driftrate model components
DriftRate = drift(A, B)
This constructor specifies the driftrate component of continuoustime
stochastic differential equations (SDEs). The driftrate specification
supports the simulation of sample paths of NVARS
state
variables driven by NBROWNS
Brownian motion sources
of risk over NPERIODS
consecutive observation periods,
approximating continuoustime stochastic processes.
The driftrate specification can be any NVARS
by1
vectorvalued function F of the general form:
$$F(t,{X}_{t})=A(t)+B(t){X}_{t}$$  (185) 
associated with a vectorvalued SDE of the form
$$d{X}_{t}=F(t,{X}_{t})dt+G(t,{X}_{t})d{W}_{t}$$
where:
X_{t} is
an NVARS
by1 state vector of process variables.
dW_{t} is
an NBROWNS
by1 Brownian motion vector.
A and B are model parameters.
The driftrate specification is flexible, and provides direct parametric support for static/linear drift models. It is also extensible, and provides indirect support for dynamic/nonlinear models via an interface. This enables you to specify virtually any driftrate specification.
Specify required input parameters as one of the following types:
A MATLAB^{®} array. Specifying an array indicates a static (nontimevarying) parametric specification. This array fully captures all implementation details, which are clearly associated with a parametric form.
A MATLAB function. Specifying a function provides indirect support for virtually any static, dynamic, linear, or nonlinear model. This parameter is supported via an interface, because all implementation details are hidden and fully encapsulated by the function.
Note: You can specify combinations of array and function input parameters as needed. Moreover, a parameter is identified as a deterministic function
of time if the function accepts a scalar time 
The required input parameters are:
A  This argument represents the parameter A.
If you specify A as an array, it must be an NVARS by1
column vector. As a deterministic function of time, when A is
called with a realvalued scalar time t as its
only input, A must produce an NVARS by1 column
vector. If you specify A as a function of time
and state, it must return an NVARS by1 column
vector when invoked with two inputs:

B  This argument represents the parameter B.
If you specify B as an array, it must be an NVARS byNVARS 2dimensional
matrix. As a deterministic function of time, when B is
called with a realvalued scalar time t as its
only input, B must produce an NVARS byNVARS matrix.
If you specify B as a function of time and state,
it must return an NVARS byNVARS column
vector when invoked with two inputs:

DriftRate  Object of class drift that encapsulates
the composite driftrate specification, with the following displayed
parameters:

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