# Model functions

## at() function

at(`f,n`)
or
@(`f,n`)

defines a binary variable which is
1 if the factor `f` has level `n ` for the record. For
example, to fit a row factor only for site 3, use the expression
at(site,3).row.
at(`f,m,n`)
or
@(`f,m,n`)

at(f,n) is extended so that
at(f,i).X at(f,j).X at(f,k).X can be written
as at(f,i,j,k).X provided
at(f,i,j,k) is written as the first component of the interaction.
Any number of levels may be listed.
at(`f`)
or
@(`f`)

at(f,i) is extended
so that at(f)
generates at(f,i) for
all levels of f.
Since this command is interpreted before the data is read, it is necessary
to declare the number of levels correctly in the field definition.
This extended form may only be used as the first term in an interaction.
le(`f,n`)

defines a binary variable which is
1 if the factor `f` has level <= `n ` for the record.
lt(`f,n`)

defines a binary variable which is
1 if the factor `f` has level < `n ` for the record.
ge(`f,n`)

defines a binary variable which is
1 if the factor `f` has level >= `n ` for the record.
le(`f,n`)

defines a binary variable which is
1 if the factor `f` has level > `n ` for the record.
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