yacc reduce/reduce conflict question

[Jeffrey W Percival]

I am using yacc (for the first time) to parse a special purpose
language, and am doing pretty good, so far.  I am confused on
something, though, and much pondering has failed to illuminate me.

I hope the following yacc output fragment is enough to sketch the problem:

279: reduce/reduce conflict (red'ns 128 and 151 ) on INCLUDE
state 279
	cmd_parameter :  MNEMONIC = MNEMONIC_    (128)
	arg :  MNEMONIC_    (151)

	.  reduce 128


Now, why is there a reduction ambiguity at the indicated spot?  I mean,
one rule has an equal sign, and the other doesn't.  Surely yacc knows
whether two things separated by '=' or a single thing has been read.

What is the problem here (or what could be my next step in reasoning it out)?
-- 
Jeff Percival (jwp at larry.sal.wisc.edu)