a = "Hello "  
b = "World"  
print("Concatenation of string a with b is ", a..b )  
print("Length of b is ",#b )  
print("Length of b is ",#"Test" )  

Concatenation of string a with b is  Hello World  
Length of b is  5  
Length of b is  4  

th> tags={{1,2,3},{3,4,4}}
th> #tags
2

th> tags={1,2,3}
                                                                      [0.0001s]
th> tags[#tags+1]=4
                                                                      [0.0001s]
th> tags
{
  1 : 1
  2 : 2
  3 : 3
  4 : 4
}