Roleset lessen.01 Verbnet Class: 1 "make less":

Roles:

Arg0:cause, agent


Arg1:Logical subject, patient, thing falling


Arg2:EXT, amount fallen


Arg3:Start point


Arg4:End point, end state of arg1


ArgM-LOC:Medium

Examples:

simple transitive (-)
Having two major products would lessen the company's vulnerability to one market's down cycle and help smooth out the flow of cash and earnings.
Arg0:	Having two major products
ArgM-MOD:	would
REL:	lessen
Arg1:	the company's vulnerability to one market's down cycle




no target for trace (-)
However, the junk-bond market has collapsed in recent weeks, lessening the likelihood that such a transaction would succeed.
Arg0:
REL:	lessening
Arg1:	the likelihood that such a transaction would succeed

I doubt the higher-numbered args will appear; for usage of those cf reduce and other verbs-of-getting-less.