Fix and test -(::Add)

Author: YingboMa

src/fast-terms.jl

@@ -98,7 +98,7 @@ end
 
 +(b::Add, a::Number) = iszero(a) ? b : Add(a + b.coeff, b.dict)
 
--(a::Add) = Add(-a.coeff, mapvalues(-, a.dict))
+-(a::Add) = Add(-a.coeff, mapvalues((_,v) -> -v, a.dict))
 
 -(a::SN) = Add(makeadd(-1, 0, a)...)
 

test/basics.jl

@@ -97,4 +97,5 @@ end
     @test repr(a+b) == "a + b"
     @test repr(-a) == "-1a"
     @test repr(-a + 3) == "3 + -1a"
+    @test repr(-(a + b)) == "-1a + -1b"
 end