Expressions of the form X@RMfixed(beta) can be used
within a formula of the type
$$response ~ fixed effects + random effects + error term$$
that specifies the Linear Mixed Model. Important Remark: RMfixed is NOT a function although
the parantheses notation is used to specify the vector of coefficients.
The matrix $X$ is the design matrix and $\beta$ is a vector of coefficients
Note that a fixed effect of the form $X$ is interpreted as
X@RMfixed(beta=NA) by default (and $\beta$ is estimated
provided that the formula is used in RFfit).
Note that the 1 in an expression 1@RMfixed(beta) is
interpreted as the identity matrix.