madness object. Note that the value and the
derivative cannot easily be changed, as allowing this form of access would
likely result in badly computed derivatives.xtag(x) <- value## S3 method for class 'madness':
xtag(x) <- value
vtag(x) <- value
## S3 method for class 'madness':
vtag(x) <- value
varx(x) <- value
## S3 method for class 'madness':
varx(x) <- value
madness object.