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 Description of tmx_param data structure.

 A tmx_param structure contains fields having the same names as all
 of the run variables and system parameters for the related mex-system.

 Some of the field names are contained in braces {}. These fields cannot
 be viewed or manipulated inside Matlab because there is no defined 
 conversion between Matlab data and tempus object types. The user can
 affect their values indirectly if the default setting expressions of
 these variables are dependent on other variables which can be modified
 inside Matlab. 

 A tmx_param may have, but does not have to have, an additional field
 called mexsysrunname which identifies the tmx_param as being
 associated with a given mex-system. This field is used purely to
 help the user keep track of tmx_param structures and their corresponding

 The values of these variables are set to one of two ways:

 1. The field can be precisely equal to a string containing the type,
    name and default setting expression of the run variable or
    parameter. This precise value is used as a flag to tell the
    mex-system that the value of the run variable or parameter
    shall be computed using its default setting expression.
 2. The field can be set to some other value which will be converted
    from a Matlab data type to a tempus object type and assigned to
    that particular parameter. It is incumbent on the user to set
    this to a value which can be converted to the corresponding
    tempus object type.

 A simple example of how this works follows. Suppose we have a mex-system
 which have three run variables and parameters combined. It does not matter
 which are run variables and which are parameters, except that run
 variables always appear before parameters, and all such variables
 appear in the order specified in the tve runset editor. Further,
 assume that we have, p, the default tmx_param returned from tmxparams,
 for a given mex-system and the field names and values are as follows:

 p.a = 'int a 5'
 p.b = 'float b a*a'
 p.c = 'float c a*b'

 If the user does not modify this tmx_param and it is used to create
 a tmx_handle using tmxcreate, the values for each of the fields
 that will be computed (see tmxcompparams) and used are:

 p.a = 5
 p.b = 25
 p.c = 125

 Suppose instead, the user modifies p such that:

 p.a = 'int a 5'
 p.b = 15
 p.c = 'float c a*b'

 and then uses p to create a tmx_handle, the following computations
 will result:

 p.a = 5
 p.b = 15
 p.c = 75

 So variables which are set to constants are not calculated. But 
 variables which continue to be set equal to the special string
 containing the default setting expression will be calculated using
 the specified values of the variables upon which they are

 These dependency rules apply to the variables whose field names
 are contained in braces. If the user affects the value of a variable
 upon which the braced variable is dependent, the computation of the
 braced variable is affected in the same way as the non-braced

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