Add 'imply' support

This is like a 'select' that only changes the default value of a symbol,
not limiting what values the user can set it to (with one exception: A
symbol implied to 'y' can't be set to 'm').

Symbol.get_implied_symbols() was added, corresponding to
Symbol.get_selected_symbols(), and Symbol.__str__() was extended to
print implied symbols and weak reverse dependencies. Weak reverse
dependencies are the 'imply' version of 'select's reverse dependencies.

1 files changed, 2 insertions, 0 deletions

diff --git a/tests/Kref b/tests/Kref
index a398c1a..a60561e 100644
--- a/tests/Kref
+++ b/tests/Kref
@@ -18,6 +18,8 @@ config MANY_REF
     depends on H
     select I if J = K || L != M
     select N if (A || !(B && (C = O)))
+    imply P if Q = R || S != T
+    imply U if (A || !(B && (C = V)))
 
 endif