better error reporting in basic::subs()

diff --git a/ginac/basic.cpp b/ginac/basic.cpp
index d5654e21739c7fd8e55e2f83208690ed66ff4f6a..ec8f2e63ffbb11b6eab2467125ec1ecd4e70abaa 100644 (file)
--- a/ginac/basic.cpp
+++ b/ginac/basic.cpp
@@ -712,7 +712,7 @@ ex basic::subs(const ex & e, unsigned options) const
                return subs(lst(e.lhs()), lst(e.rhs()), options);
        }
        if (!e.info(info_flags::list)) {
-               throw(std::invalid_argument("basic::subs(ex): argument must be a list"));
+               throw(std::invalid_argument("basic::subs(ex): argument must be a relation_equal or a list"));
        }
 
        // Split list into two