public void enterOuterAlt(ParserRuleContext localctx, int altNum) {
		localctx.setAltNumber(altNum);
		// if we have new localctx, make sure we replace existing ctx
		// that is previous child of parse tree
		if ( _buildParseTrees && _ctx != localctx ) {
			ParserRuleContext parent = (ParserRuleContext)_ctx.parent;
			if ( parent!=null )	{
				parent.removeLastChild();
				parent.addChild(localctx);
			}
		}
		_ctx = localctx;
	}
Always called by generated parsers upon entry to a rule. Access field
{@link #_ctx} get the current context.
runtime/Java/src/org/antlr/v4/runtime/Parser.java