[R] User defined covariate structure.

[jswansmi at uoguelph.ca]

I am trying to use nlme but instead of using one of the “identity” variance or
covariance matrixes such as compsymm or ar1.  Instead I want the covariance
matrix to be represented in the following manor.  Is it possible to define my
own covariance matrix?
I have search and found papers saying I can define my own covariance matrixes
and own correlation structures.  Said use corstruct but not sure how to
implement it.  Also found documentation to use re.structur.  If able to help me
out it be greatly appreciated as I am stuck.

|1	p1g	p2g	p3g	p4g	
|
|p1g	1	p1g	p2g	p3g	
|
|p2g 	p1g	1	p1g	p2g	
|
|p3g	p2g	p1g	1	p1g 	
|
|:	:	:	:	:	
|