Pipeline rule language parsing error

The grammar defined for ANTLR to recognize a nested expression fails when there are no whitespaces and an arithmetic operation is part of it, for example, function(a,function(b)-1). In this case, -1 is recognized as a SignedExpression, which leads to parsing failure since function SignedExpression is not a valid expression."

## Expected Behavior

Either we improve our documentation or improve the grammar for now we have multiple workarounds.
## Steps to Reproduce (for bugs)

Create a rule using the editor:
`````
rule "test"
when
true
then
let timeday = "011";
let timeday = substring(timeday,length(timeday)-2);
end

`````
## Context

gz#221