l.y and d.y

Dear colleagues:
I met parameters that I cannot found in help.
They are: l. and d.

part of the code:
"ivreg d.y d.x (l.d.y= l2.y) if t>1000-`z'
return scalar gamma`z'_iv=_b[l.d.y]
return scalar beta`z'_iv=_b[d.x]"

Please, explain what they are used for. :D
L, D, F are time-series operators.

L = lag
D = difference

.help operator for more info (click 13 Functions and expressions) --> page 108