search for difference and find the above line

hello All,

i have to two files package.today and package.yesterday , the extension of the files says what day the file belongs to .
contents of the file change little bit everyday .

i am taking only the package and sub package from the files

awk '/^[0-9+]/{print $0}' Packages.today
82                   
83                   
84                   
85                   
86                   
+--- 21
+--- 82
87                   
88                   
89                   
90                   
900                  
901                  
903                  
904                  
905                  
909                  
91                   
+--- 11
+--- PREVIEW PACKAGE
92                   
93                   
94                   
95                   
950                  
96                   
97                   
98                   
+--- 11
99

awk '/^[0-9+]/{print $0}' Packages.yesterday
82                   
83                   
84                   
85                   
86                   
+--- 21
+--- 82
87                   
88                   
89                   
90                   
900                  
901                  
903                  
904                  
905                  
909                  
91                   
+--- 11
92                   
93                   
94                   
95                   
950                  
96                   
97                   
98                   
+--- 11
99

The main package starts from the line begining and sub package begins with +--- , both the package names can be number and char

the difference between yesterdays and todays package list is

diff tmp.today tmp.yesterday | grep +--- | sed 's/<//g;s/+---//g;s/^ *//g'
PREVIEW PACKAGE

so "preview package" is a new sub package added in under the main package 91

i want to know the number what main package is the new sub package added ( here its is 91)

i may be following a longer path using diff command , all i want to know the main package number the sub package added,