Maybe inconsistent behavior of the method compute_drift() in VPCPlan and ICPlan ?

[zhengyunqq]

In the ICPlan, compute_drift() returns (-drift, diffusion).

However, in VPCPlan, I think compute_drift() returns (drift, diffusion).
drift = d_alpha_alpha_ratio * x, for VPCPlan, d_alpha_alpha_ratio= -0.5 * \beta
but compute_drift() in VPCPlan returns -0.5*\beta instead of 0.5*\beta

Their signs on drift are incosistent.