2 CONSTRAIN
  Syntax:   CONSTRAIN par expression
  where:    par         is a MINUIT parameter number
            expression  is an arithmetic expression

  Constrains the value  of a parameter that is  being fit. The parameter
  is  fixed and  its  value  is  calculated  every   iteration  from the
  constraint. The constraint can be  any arithmetic expression
  (see HELP Expressions for  details). The most  common use is to fix 2
  parameters relative to  each other. You can  remove a constraint using
  the `UNCONSTRAIN' command. You can  list the current constraints using
  the `SHOW CONSTRAINT' command.

  You can specify other parameters that are being fit using the syntax
  `Pn' (where `n' is the MINUIT parameter number), or `Pn(m)' (where `n'
  is the function number and `m' is the parameter number in the function).

  WARNING: The  constraint is  parsed at the time it  is defined and is
  calculated before the loop over  the points being fit is done. Thus if
  you use  registers in  the  constraint their  contents at  the time of
  minimization (i.e. when you give the `MINIMIZE' command) will be used.

  This command is only available in the CERN version of MINUIT.

3 Examples

!\begin{enumerate}
!\item
!^
  Example 1:
   Constrain the mean and sigma of 1 Gaussian with respect to a second:
    fun add gaus sig             !Add 2 Gaussians
    1000
    1
    0.5
    fun add gaus sig
    1000
    2
    0.5
    fit 1
    0
    !Constrain the mean of the second Gaussian to be 1 > the first
    !Use the MINUIT parameter number
    constrain 5 p2+1
    !Constrain the sigma of the second Gaussian to be 2x that of the first
    !Use the Mn_Fit parameter number
    constrain 6 2*p1(3)
    minimize
    show constrain
    unconstr 5                  !Remove the constraint on parameter 5
    minimize

!\end{enumerate}


