Multiple references separated by semi-colon

Question

The following is in the preamble I "inherited":

\documentclass[a4paper,12pt]{book}
\usepackage{amsmath, amsthm, amssymb}
\usepackage{fancyhdr}
\pagestyle{fancy}
\renewcommand{\headrulewidth}{0.1pt} % for upper line
\renewcommand{\footrulewidth}{0.1pt} % for lower line
\fancyhead[LE,RO]{\itshape \nouppercase \rightmark}
\fancyhead[LO,RE]{\itshape \nouppercase \leftmark}
\fancyfoot[C]{\thepage}

\usepackage{fancybox}

\usepackage{xcolor}
\usepackage{framed}

\numberwithin{equation}{section} % numbering according to the section
\bibliographystyle{plain} %Choose a bibliograhpic style

\usepackage{color}
\usepackage[colorlinks=true,linkcolor=red,citecolor=blue,urlcolor=black{hyperref}
\setlength{\headheight}{15pt}

%\parindent0em
\parskip1ex

In my .tex file, for each reference I state, for instance: "as reported in \citep{schum98,berg90,tom90}". The pdf output "[Schuman, 1998, Berg, 1990, Tomaven, 1990]". As you may notice, such a listing is awkward. How could I separate the data belonging to a specific author by a semi-colon from the data belonging to other authors, as in "[Schuman, 1998; Berg, 1990; Tomaven, 1990]". Note that a large portion of the .tex document has been written, and I would prefer the option of just changing the preamble.

Answer 1

I solved it by adding \usepackage[semicolon]{natbib}. Thanks for pointing out the missing natbib.