The fifth and final volume of the Dictionary of Regional American English has finally been published, more than 50 years after the project began, and contained in the latest volume, which covers Sl-Z, are the first definitions for “soda’’ and “tonic,’’ plus maps showing where those terms are used. That map looks a lot like I-495 in Massachusetts. I’ll be focusing on the town of Mendon, just off 495, where respondents used both terms, to find the tonic/soda line.