Appending a running number to the parser output in ANTLR - concatenation

Question

I'm quite new to ANTLR and trying to concatenate a number incremented automatically to my parser output. My grammar looks like below. I defined the variable mtCount to store my increment values. The file gets successfully parsed and cpformat includes everything intended but the value of mtCount is not concatenated to the output. Any ideas, what I am doing wrong?
grammar CP3;
#parser::members {
public int mtCount = 0;
}
file returns [String cpformat]
: 'type' t=type {$cpformat += $t.text;}
'variable' v=variable {$cpformat += $v.text;}
'rule' rs=rulestr {$cpformat += $rs.text;}
;
type returns [String cpformat]
: (dn=domname {$cpformat += $dn.text;}
'{' vv1=varval {$cpformat += $vv1.text;}
(',' vv2=varval {$cpformat += $vv2.text;} )* '};')*
;
variable returns [String cpformat]
: (dn=domname {$cpformat += $dn.text;}
vn=varname {$cpformat += $vn.text;} ';')*
;
rulestr returns [String cpformat]
: ('(' '('
mt1=minterm {$cpformat += $mt1.text + " mtCount: " + mtCount++;}
('||' mt2=minterm {$cpformat += $mt2.text + " mtCount: " + mtCount++;})*
')' ')' ';')*
;
minterm returns [String cpformat]
: '(' vn1=varname {$cpformat += $vn1.text;}
'==' vv1=varval {$cpformat += $vv1.text;}
('&&' vn2=varname {$cpformat += $vn2.text;}
'==' vv2=varval {$cpformat += $vv2.text;} )* ')'
;
domname returns [String cpformat]
: str1=STRING {$cpformat = $str1.text;}
;
varname returns [String cpformat]
: str2=STRING {$cpformat = $str2.text;}
;
varval returns [String cpformat]
: str3=STRING {$cpformat = $str3.text;}
;
STRING : ('a'..'z'|'A'..'Z'|'0'..'9'|'_')+ ;
WS : ( ' ' | '\t' | '\r' | '\n' )+ {skip();} ;
Neither mtCount, nor its value can be seen in the output. The whole output is a bit lengthy, but I send a part of it below, it should appear in this part of the output for example:
dom97var97;dom98var98;dom99var99;dom100var100;dom101var101;
((
Here, I would expect it.
(var1==S64&&var3==M9&&var81==BVM5&&var94==EU93&&var95==CRIT1870CC&&var96==CRIT095CV&&var99==JC5&&var100==CRIT784&&var101==F8Q)
||(var1==S64&&var3==M9&&var81==BVM5&&var94==EU93&&var95==CRIT1870CC&&var96==CRIT095CV&&var99==JC5&&var100==CRIT786&&var101==F8Q)
||(var1==S64&&var3==MA&&var81==BVM5&&var94==EU96&&var95==CRIT1870CC&&var96==CRIT065CV&&var99==JB1&&var100==CRIT620&&var101==F8Q)
||(var1==S64&&var3==MA&&var81==BVM5&&var94==EU96&&var95==CRIT1870CC&&var96==CRIT065CV&&var99==JB1&&var100==CRIT624&&var101==F8Q)
|| ...